<PAGE>   1
                          CAPACITY COMMITMENT AGREEMENT


             THIS CAPACITY COMMITMENT AGREEMENT (as amended, supplemented or
otherwise modified from time to time, this "Agreement"), entered into as of this
10 day of November, 2000, between SOUTH AMERICAN CROSSING LTD. a company
organized and existing under the laws of Bermuda and having its principal office
in Hamilton, Bermuda (said company, and any permitted successor or assign
hereunder, ("Grantor") and IMPSAT FIBER NETWORKS, INC. a corporation organized
and existing under the laws of the State of Delaware having its principal office
at Wilton Manors, Florida (said company, and any permitted successor or assign
hereunder, "Purchaser"). Grantor and Purchaser are herein sometimes collectively
referred to as the "Parties" and each, individually, as a "Party".

                              W I T N E S S E T H:

             WHEREAS, Mid-Atlantic Crossing Ltd. has constructed a fiber optic
cable system connecting New York, Florida and St. Croix, known as the
Mid-Atlantic Crossing System or "MAC-1";


             WHEREAS, Pan American Crossing Ltd. is constructing a fiber optic
cable system connecting California, Mexico, Panama and St. Croix, known as the
Pan American Crossing System or "PAC";


             WHEREAS, Grantor is constructing a fiber optic cable system
connecting various principal cities in South America, known as the South
American Crossing System or "SAC";

             WHEREAS, Atlantic Crossing Ltd. has constructed fiber optic cable
systems connecting the United States, the United Kingdom, the Netherlands and
Germany known as Atlantic Crossing System, I and II or "AC-1"; and "AC-2"

             WHEREAS, Pacific Crossing Ltd. is constructing a fiber optic cable
system connecting the United States and Japan known as the Pacific Crossing
System or "PC-1";


             WHEREAS, Global Crossing Pan European Crossing Holdings B.V. is
constructing a fiber optic cable network connecting various principal cities in
Europe, known as Pan European Crossing or "PEC";


             WHEREAS, Mid-Atlantic Crossing Ltd., Pan American Crossing Ltd.,
South American Crossing Ltd., Atlantic Crossing Ltd, Pacific Crossing Ltd, and
Global Crossing Pan European Crossing Holdings B.V. and each of their respective
subsidiaries, are affiliates hereinafter collectively referred to as the "System
Companies" and MAC-1, PAC, AC-1, AC-2, PC-1 and PEC SAC are hereinafter
collectively referred to as the "Systems", (attached hereto as Exhibit 1 is a
map of the Systems);

             WHEREAS, additional companies under common control with Grantor may
in the future construct other systems, though there is no obligation to do so,
in which case such companies will be deemed to be System Companies under this
Agreement; and such systems will be deemed


<PAGE>   2

to be "Systems"

             WHEREAS, Purchaser desires to acquire rights with respect to
capacity on one or more of the Systems on an indefeasible right of use basis
("IRU"); and

             WHEREAS, Grantor is affiliated with each of the System Companies
and can cause IRUs to be granted in capacity on the Systems to Purchaser;

             NOW, THEREFORE, the Parties, in consideration of the mutual
covenants contained herein, and for other good and valuable consideration, the
receipt of which is hereby acknowledged, covenant and agree with each other as
follows:

1.     DEFINITIONS.

       Unless otherwise defined herein, all terms which are commonly used in the
telecommunications industry shall have the meanings commonly given such terms in
such industry. In addition to terms defined in the preamble, the recitals, and
in the text of this Agreement, the following terms shall have the following
meanings:

       "Affiliate" means in relation to any entity any other present or future
       entity controlled by or under common control with such entity. As used in
       this definition "control" shall mean as to any Person, (i) any
       corporation of which more than 20% of the outstanding stock of any class
       or classes having by the terms thereof ordinary voting power to elect a
       majority of the directors of such corporation (irrespective of whether or
       not at the time stock of any class or classes of such corporation shall
       have or might have voting power by reason of the happening of any
       contingency) is at the time owned by such person and/or one or more
       Subsidiaries of such person and (ii) any partnership, association, joint
       venture or other entity in which such person and/or one or more
       Subsidiaries of such person have more than a 20% equity interest therein.

       "Annual Commitments" means the amount of Capacity that must be purchased
       prior to the end of an Annual Period beginning on the Effective Date.

       "Annual Period" means, for the first such period, the period beginning on
       the Effective Date and ending on the [ ]* of the RFS Date and,
       thereafter, [ ]* period occurring during the term of this Agreement.

       "Availability Date" means the date on which the Capacity is available for
       service in accordance with the ITU recommendations specified in the
       Capacity Purchase Agreement.

       "Anchor Tenant Prices" means the prices set out in Section 3.



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* CONFIDENTIAL TREATMENT REQUEST - Confidential portion has been omitted and
filed separately with the Commission.

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       "Business Day" means a day other than a Saturday, Sunday or other day on
       which commercial banks in Bermuda, Buenos Aires or New York City are
       authorized or required to close.

       "Capacity" means capacity on the Systems.

       "Capacity Purchase Agreement" or "CPA" means the agreement of even date
       entered into by the Parties or other agreements to be entered into
       between the Parties or their Affiliates pursuant to which Purchaser or
       its Affiliates shall acquire and Grantor or its Affiliates shall grant
       any IRUs contemplated by this Agreement (attached hereto as Appendix A).

       "Dollars" or "$" means United States Dollars.


       "Effective Date" means the date this Agreement has been signed by both
       Parties.

       "Indefeasible Right of Use" or "IRU" means a contractual right for a
       defined duration of years in which the Purchaser or its Affiliate is
       authorized to utilize certain Capacity of the Grantor, and which
       contractual right of utilization is not subject to defeasance, denial or
       withdrawal except on terms stated in the Agreement or the applicable
       Capacity Purchase Agreement.

       "Minimum Capacity Unit" or "MCU" means, with respect to any System, the
       minimum amount of capacity that may be purchased by the Purchaser on such
       System. The Grantor shall designate the MCU on each System.

       "Points of Presence" or "POP" means the points of presence of the Grantor
       and/or its Affiliates on the Global Crossing Network at which Purchaser
       is authorized by Grantor to interconnect to send and receive traffic to
       and from the Global Crossing Network. Individually, each of the Points of
       Presence may be referred to herein as a "Point of Presence".

       "Published Prices" means the Global Crossing list prices for Capacity
       that Grantor generally offers to its Customers for the purchase of single
       MCUs on the Systems as will be supplemented from time to time to include
       new systems and reflect any price changes.

       "RFS Date" means the date upon which the Grantor notifies the Purchaser
       in writing that SAC as described in Exhibit 1 is ready for service on a
       fully protected basis.

       "Subsidiary" means as to any Person, (i) any corporation of which more
       than 20% of the outstanding stock of any class or classes having by the
       terms thereof ordinary voting power to elect a majority of the directors
       of such corporation (irrespective of whether or not at the time stock of
       any class or classes of such corporation shall have or might have voting
       power


                                       3
<PAGE>   4

       by reason of the happening of any contingency) is at the time owned by
       such person and/or one or more Subsidiaries of such person and (ii) any
       partnership, association, joint venture or other entity in which such
       person and/or one or more Subsidiaries of such person have more than a
       20% equity interest therein.

       "SAC Wavelength" means a 10 gigabits per second Wavelength on SAC with
       access to the Points of Presence at Puerto Viejo, Venezuela; Fortaleza,
       Brazil; Rio de Janeiro, Brazil; Santos, Brazil and Buenos Aries,
       Argentina.

       "Wavelength" means a linear bi-directional 2.5 gbps or 10 gbps
       wavelength.

2.     PURCHASE AGREEMENT.

       (a)   The Purchaser hereby unconditionally and irrevocably agrees to
             purchase, pay for and activate, and/or cause one or more of its
             Subsidiaries, to purchase, pay for and activate, MCUs on the
             Systems in an aggregate amount of $46,000,000 (the "Commitment")
             during the period commencing on the Effective Date and ending [ ]*
             after the RFS Date (the "Capacity Purchase Period"). The Parties
             acknowledge that the Initial Committed Capacity purchased under the
             CPA of even date shall be applied towards Purchaser's Annual
             Commitment.

             [  ]*

       (b)   The payment terms for purchases of MCU's in accordance with the
             terms hereof shall be as follows:

             (i)    The Purchaser shall pay the sum of $[ ]* as a non-refundable
                    deposit (the "Deposit"), representing [ ]*% of the
                    Commitment, within three Business Days of the date of this
                    Agreement .The Deposit shall be allocated on a pro rata
                    basis towards the Initial Payment of all Capacity purchased
                    hereunder.

             (ii)   The Purchaser shall pay the Purchase Price for each MCU of
                    Capacity purchased during the Capacity Purchase Period (less
                    [ ]*% representing the pro rata portion of the Deposit
                    allocated to the relevant Capacity) immediately prior to
                    activation of that Capacity in accordance with Section 3 of
                    the CPA. In satisfaction of the Commitment, Purchaser shall
                    purchase Capacity on the Systems at least equal to the
                    amounts set out below, by the dates set out below. If
                    Purchaser fails to purchase and pay for such Capacity prior
                    to the dates set out below the Purchaser shall pay the
                    amount of the deficit in the amount purchased during the
                    applicable Annual Period on the dates set out below.
                    Capacity purchases in excess of an Annual Commitment


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* CONFIDENTIAL TREATMENT REQUEST - Confidential portion has been omitted and
filed separately with the Commission.


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<PAGE>   5

                    shall be applied to reduce the amount of the Annual
                    Commitment due in the succeeding Annual Period.

             (iii)  The Deposit and payments applied towards the Annual
                    Commitment shall be non-refundable and shall be credited
                    towards the payment of the Purchase Price for Capacity
                    purchased to satisfy the Commitment.



                                                          Annual Commitment
               Date
               ----
               [   ]*




       (d)    Grantor shall cause its Affiliates to comply with the terms of the
              CPA and make Capacity available on an IRU basis on the Systems.
              Purchaser's and Grantor's Affiliates have executed a CPA on the
              Effective Date which, subject to Section 2 (e), shall govern all
              future purchases of Capacity to fulfill the Commitment. Each
              purchase of MCUs on any System pursuant to Section 2(a) of this
              Agreement shall be effected by the Purchaser or its Affiliate(s)
              executing, delivering and complying with a supplement to the CPA
              in the form of Schedule 2 to that CPA or as otherwise required by
              Section 2(e).

       (e)    At the request of either party within 30 days of any Capacity
              being purchased the parties shall cause their local Affiliate(s)
              to execute separate capacity purchase agreements , for sections of
              the Capacity situated within the territorial limits of each of the
              countries in South America to which Capacity is being sold under
              the CPA and this Agreement ("Local Capacity"), which shall be
              executed by the aforementioned entities within five business days
              (5) of the date of such request. Such separate agreements shall be
              on terms identical to those stated in the CPA , except as to
              modifications which may be required to conform to local law.

       (f)    [ ]*

       (g)    [ ]*

       (h)    [ ]*. If the Purchaser does not enter into an agreement to
              purchase such Capacity within 30 days of such notice

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* CONFIDENTIAL TREATMENT REQUEST - Confidential portion has been omitted and
filed separately with the Commission.


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<PAGE>   6

              then Grantor shall have no further obligation to make such
              Capacity available to Purchaser until such time as Grantor
              completes the relevant upgrade or construction of a new System
              which may be required to make additional Capacity available.

       (i)    The unavailability of Capacity following proper notice as
              indicated in section 2(h) shall not excuse Purchaser from paying
              an amount equal to the Annual Commitments on the dates set out in
              Section 2(c); however if Grantor does not make new Capacity
              available within six months of the date Capacity becomes
              unavailable on a System or traffic route, and such unavailability
              has prevented Purchaser from activating sufficient Capacity on the
              Systems to meet its annual Commitment, then Grantor may at its
              discretion either extend the next Annual Payment due date or allow
              Purchaser to delay payment of a portion of the Annual Payment as
              applicable.

       (j)    The Grantor will at any time upon the Purchaser's request, but not
              more than once per month, supply the Purchaser with its current
              list of Published Prices.

3.     ANCHOR TENANT PRICES

The Anchor Tenant Prices for POP to POP connections are as follows: -

(a)     SAC


       (i)    $[ ]* per STM-1 between any two Grantor's POPs located on SAC as
              set forth in Exhibit 2; with Initial Maintenance Costs per STM-1
              per annum as follows: In year 1, $[ ]*, in year 2, $[ ]* and in
              year 3 and each subsequent year until termination of the IRU, $[
              ]*.

       (ii)   $[ ]* per STM-1 between any of Grantor's POPs located on SAC and
              Miami, Florida or New York, New York; and

       (iii)  $[ ]* per STM-1 between Caracas Venezuela or Panama and Miami,
              Florida or New York, New York, with Initial Maintenance Costs per
              STM-1 per annum as follows: In year 1 $[ ]* 1; year 2 $[ ]*; and
              year 3 and each subsequent year until termination of the IRU $[
              ]*.

       (iv)   The prices described above reflect the [ ]* described in Section 2
              (b) on the specified POP to POP connections for Capacity purchased
              to satisfy the Commitment.

(b)     PAC

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* CONFIDENTIAL TREATMENT REQUEST - Confidential portion has been omitted and
filed separately with the Commission.

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<PAGE>   7

              Purchaser [ ]* in effect [ ]* days before the date of Activation
              of Capacity on any POP to POP connection between any South
              American City on SAC and Los Angeles, California, or Mexico City,
              Mexico.

       (c)    OA&M

              The maintenance costs for the Capacity purchased pursuant to this
              Agreement will be a flat rate per annum payable quarterly in
              advance. Maintenance costs will be as agreed between the parties
              as set out in Schedule 1 of the CPA. For all future Capacity
              purchases, maintenance costs will not exceed [ ]*. Maintenance
              costs for Capacity purchased on the Excluded Systems will be the
              Published Prices for such costs at the date of the relevant CPA.

4.     COLLOCATION

       The Grantor agrees to provide collocation space to Purchaser for the
installation of equipment at any POP on substantially the same terms specified
in the License Agreement attached hereto as Appendix A, which may be modified
when necessary to conform to the requirements of local law. The Purchaser will
be permitted to connect to the ODF interconnected to the SDH platform at the
Facility (as defined in the Collocation Form attached as Appendix A) or, where
this is not permitted under the terms of the Licence Agreement , the Purchaser
shall require the Grantor to carry out any necessary work to connect to the ODF
on behalf of and at the cost of the Purchaser. This provision shall not operate
so as to provide any limitations in connection with future collocation of
Wavelengths.

5.     REPRESENTATIONS

       (a)   The Grantor hereby represents and warrants to the Purchaser that
(i) the Grantor is a corporation duly organized and validly existing under the
laws of Bermuda; (ii) the execution, delivery and performance of this Agreement
by Grantor has been duly authorized by all necessary corporate action on the
part of Grantor and this Agreement is a valid, binding and enforceable
obligation of Grantor enforceable with its terms and (iii) the execution,
delivery and performance of this Agreement by Grantor does not violate, conflict
with or constitute a breach of, the organizational documents or any order,
decree or judgment of any court, tribunal or governmental authority binding on
Grantor (iv) that Grantor has the authority to cause its Affiliates and
Subsidiaries to make Capacity available and grant IRUs to Purchaser on the
System .

       (b)   The Purchaser hereby represents and warrants to the Grantor that
(i) the Purchaser is a corporation duly organized and validly existing under the
laws of the State of Delaware; (ii) the execution, delivery and performance of
this Agreement by the Purchaser has been duly authorized by all necessary
corporate action on the part of such Purchaser and this Agreement is a valid,
binding


----------------
* CONFIDENTIAL TREATMENT REQUEST - Confidential portion has been omitted and
filed separately with the Commission.


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<PAGE>   8

and enforceable obligation of the Purchaser enforceable in accordance with its
terms; and (iii) the execution, delivery and performance of this Agreement by
the Purchaser does not violate, conflict with or constitute a breach of, the
organizational documents or any order, decree or judgment of any court, tribunal
or governmental authority binding on the Purchaser.

6.      SETTLEMENT OF DISPUTES.

       (a)   The parties hereto shall endeavor to settle amicably by mutual
discussions any disputes, differences, or claims whatsoever related to this
Agreement.

       (b)  Failing such amicable settlement, any controversy, claim or dispute
arising under or relating to this Agreement, including the existence, validity,
interpretation, performance, termination or breach thereof, shall finally be
settled by arbitration in accordance with the International Arbitration Rules of
the American Arbitration Association ("AAA"). There shall be three (3)
arbitrators (the "Arbitration Tribunal"), the first of which shall be appointed
by the claimant in its notice of arbitration, the second of which shall be
appointed by the respondent within thirty (30) days of the appointment of the
first arbitrator and the third of which shall be jointly appointed by the
party-appointed arbitrators within thirty (30) days thereafter. The language of
the arbitration shall be English. The Arbitration Tribunal shall issue a written
opinion and will not have authority to award punitive damages to either party.
Each party shall bear its own expenses, but the parties shall share equally the
expenses of the Arbitration Tribunal and the AAA. This Agreement shall be
enforceable, and any arbitration award shall be final, and judgment thereon may
be entered in any court of competent jurisdiction. The arbitration shall be held
in New York, New York, USA.

7.      GOVERNING LAW.

       THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK, UNITED STATES OF AMERICA.

8.     NO THIRD PARTY BENEFICIARIES; WAIVER OF IMMUNITY.

       This Agreement does not provide and is not intended to provide third
parties (including, but not limited to, customers of the Purchaser) with any
remedy, claim, liability, reimbursement, cause of action, or any other right.
The parties hereto acknowledge that this Agreement is commercial in nature, and
each party hereto expressly and irrevocably waives any claim or right which it
may have to immunity (whether sovereign immunity, act of state or otherwise) for
itself or with respect to any of its assets in connection with an arbitration,
arbitral award or other proceeding to enforce this Agreement, including, without
limitation, immunity from service of process, immunity of any of its assets from
pre- or post-judgment attachment or execution and immunity from the jurisdiction
of any court or arbitral tribunal.

9.     ASSIGNMENT.

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<PAGE>   9

       (a)   This Agreement and all of the provisions hereof shall be binding
upon and inure to the benefit of the parties hereto and their respective
successors and permitted assigns.

       (b)   The Grantor shall solely be responsible for complying with all of
the terms binding on the "Grantor" hereunder and shall not be permitted to
assign, transfer or otherwise dispose of any or all of its right, title or
interest hereunder or delegate any or all of its obligations hereunder to any
person or entity except that the Grantor shall be permitted to (i) effect a
collateral assignment of its rights hereunder to one or more lenders to the
Grantor or its Affiliates and (ii) assign, transfer or otherwise dispose of any
or all of its rights hereunder and delegate any or all of its obligations
hereunder to any Affiliate or Subsidiary of the Grantor. The Grantor shall give
the Purchaser notice of any such assignment, transfer or other disposition or
any such delegation.

       (c)   The Purchaser shall solely be responsible for complying with all of
the terms binding on the "Purchaser" hereunder and shall not be permitted to
assign, transfer or otherwise dispose of any or all of its right, title or
interest hereunder or delegate any or all of its obligations hereunder to any
person or entity; provided, that the Purchaser may assign its right to: (i)
effect a collateral assignment of its rights hereunder to one or more lenders to
the Grantor or its Affiliates and (ii) assign, transfer or otherwise dispose of
any or all of its rights hereunder and delegate any or all of its obligations
hereunder to any Affiliate or Subsidiary. The Purchaser shall give the Grantor
prior written notice of any such assignment

       (d)   Any assignment, transfer or other disposition by any party hereto
which is in violation of this Section shall be void and of no force and effect.

10.    NOTICES.

       Each notice, demand, certification or other communication given or made
under this Agreement shall be in writing in English and shall be delivered by
hand or sent by registered mail or by facsimile transmission to the address of
the respective party as shown below (or such other address as may be designated
in writing to the other party hereto in accordance with the terms of this
Section):

                 If to the Purchaser:  IMPSAT Fiber Networks, Inc.
                                       E. Rawson de Delleplane # 150 9th Floor
                                       (C1107BCA)
                                       Buenos Aires
                                       Argentina
                                       Fax No: 5411 4363 3758
                                       Attention: Alexander Rivelis

                 If to the Grantor:    South American Crossing Ltd.
                                       Wessex House
                                       45 Reid Street
                                       Hamilton HM 12 Bermuda
                                       Fax No.: 1 441 296 8607
                                       Attention: General Counsel

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<PAGE>   10

       Any change to the name, address and facsimile numbers may be made at any
time by giving fifteen (15) days prior written notice in accordance with this
Section. Any such notice, demand or other communication shall be deemed to have
been received, if delivered by hand, at the time of delivery or, if posted, at
the expiration of seven (7) days after the envelope containing the same shall
have been deposited in the post maintained for such purpose, postage prepaid,
or, if sent by facsimile, at the date of transmission if confirmed receipt is
followed by postal notice.

11.    SEVERABILITY.

        If any provision of this Agreement is found by an arbitral, judicial or
regulatory authority having jurisdiction to be void or unenforceable, such
provision shall be deemed to be deleted from this Agreement and the remaining
provisions shall continue in full force and effect.

12.    HEADINGS

       The Section headings of this Agreement are for convenience of reference
only and are not intended to restrict, affect or influence the interpretation or
construction of provisions of such Section.

13.    COUNTERPARTS.

       This Agreement may be executed in counterparts, each of which when
executed and delivered shall be deemed an original. Such counterparts shall
together (as well as separately) constitute one and the same instrument.

14.    ENTIRE AGREEMENT

       This Agreement supersedes all prior or written understandings between
the parties hereto and, together with the Shareholders Agreement, constitutes
the entire agreement with respect to the subject matter herein. This Agreement
shall not be modified or amended except by a writing signed by authorized
representatives of the parties hereto.

15.    PUBLICITY AND CONFIDENTIALITY.

       The provisions of this Agreement and any non-public information, written
or oral, with respect to this Agreement ("Confidential Information") will be
kept confidential and shall not be disclosed, in whole or in part, to any person
other than Affiliates, officers, directors, employees, agents or representatives
of a party (collectively, "Representatives") who need to know such Confidential
Information for the purpose of negotiating, executing and implementing this
Agreement. Each party agrees to inform each of its Representatives of the
non-public nature of the Confidential Information and to direct such persons to
treat such Confidential Information in

                                       10
<PAGE>   11

accordance with the terms of this Section. Nothing herein shall prevent a party
from disclosing Confidential Information (i) upon the order of any court or
administrative agency, (ii) as required by law or upon the request or demand of,
or pursuant to any regulation of, any regulatory agency or authority, (iii) to
the extent reasonably required in connection with the exercise of any remedy
hereunder, (iv) to a party's legal counsel or independent auditors, (v) to
prospective lenders to the Grantor, (vi) to the extent necessary, to the
operator, maintainer and administrator of any System and/or (vii) to any actual
or proposed permitted assignee of all or part of its rights hereunder provided
that such actual or proposed assignee agrees in writing to be bound by the
provisions of this Section. Notwithstanding the foregoing, in the event that the
Purchaser intends to disclose any Confidential Information pursuant to clause
(i) or (ii) of the preceding sentence, the Purchaser agrees to (a) provide the
Grantor with prompt notice before such disclosure in order that the Grantor may
attempt to obtain a protective order or other assurance that confidential
treatment will be accorded such Confidential Information and (b) cooperate with
the Grantor in attempting to obtain such order or assurance.

16.    LIMITATION OF LIABILITY.

       In no event shall the Purchaser or the Grantor (or any of its
Affiliates) be liable to the other for consequential, incidental, indirect or
special damages, including, but not limited to, loss of revenue, loss of
business opportunity, or the costs associated therewith. The terms of the CPA
shall govern Grantor's liability in respect to Capacity purchased there under.


             IN WITNESS WHEREOF, the parties hereto have executed this
Agreement in the jurisdictions set forth beneath their signatures, effective on
the date first written above.


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<PAGE>   12





SOUTH AMERICAN CROSSING LTD.


By:
       ------------------------------------------------------------
        Name:
        Title:



IMPSAT FIBER NETWORKS, INC.


By:
       ------------------------------------------------------------
        Name:
        Title:



                                       12

<PAGE>   13

                             [GLOBAL CROSSING LOGO]



                          CAPACITY COMMITMENT AGREEMENT

                                     BETWEEN

                          SOUTH AMERICAN CROSSING LTD.

                                       AND

                           IMPSAT FIBER NETWORKS, INC.



                   ------------------------------------------



                                      -13-