AMENDMENT NO. 6 TO ESCROW AGREEMENT

            THIS AMENDMENT NO. 6 to the Escrow Agreement is made as of the 14th
day of May, 2003 (as supplemented or modified from time to time, this
"Agreement") by and among AJG Financial Services, Inc., through its
Vice-President, General Counsel as agent (the "Major Shareholder Agent") for
Bernard J. Zahren, Finova Mezzanine Capital Corp., AJG Financial Services, Inc.,
Environmental Opportunities Fund (by itself and as successor to Environmental
Opportunities Fund Cayman), Fredric Rose, M&R Associates, Martin F. Laughlin,
Richard C. Augustine and Michael J. Carolan (each, an "Initial Major
Shareholder") and for Stephen Rosenberg, U.S. Energy Systems, Inc., a Delaware
corporation ("USE"), and US Energy Biogas Corp. (formerly known as Zahren
Alternative Power Corporation), as the successor to USE Acquisition Corp. a
Delaware corporation (the "Sub" and together with USE, the "USE Parties"),
Cinergy Energy Solutions, Inc., a Delaware corporation ("CES" and together with
the USE Parties, the "Beneficiaries"), and Tannenbaum Helpern Syracuse &
Hirschtritt LLP having an office at 900 Third Ave., New York, New York, 10022
(the "Escrow Agent"). Unless indicated otherwise, capitalized terms shall have
the same meanings herein as they have in the Escrow Agreement (as defined
below).

                              W I T N E S S E T H:

            WHEREAS, the parties hereto and the Initial Major Shareholders have
previously entered into that certain Escrow Agreement dated as of November 28,
2000 (as amended by Amendment No. 1 dated as of May 11, 2001, Amendment No. 2
dated as of November 1, 2002, Amendment No. 3 dated as of February 10, 2003,
Amendment No. 4 dated as of March 13, 2003, and Amendment No. 5 dated as of
April 15, 2003 the "Escrow Agreement");

            WHEREAS, the parties hereto now wish to amend the Escrow Agreement;

            NOW, THEREFORE, in consideration of $10.00 and other consideration,
the receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

            1.    The last sentence of Section 3(d) of the Escrow Agreement is
                  hereby amended by deleting the existing sentence in its
                  entirety and inserting in its place the following sentence:

                  The "Release Date" shall be June 15, 2003.

            2.    Except as amended hereby, the Escrow Agreement is hereby
                  ratified and confirmed and, as so amended, remains in full
                  force and effect on the date hereof.


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            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be Executed as of the date first written above.

                                        MAJOR SHAREHOLDER AGENT:
                                        AJG FINANCIAL SERVICES, INC.

                                        By: ____________________________________
                                            Name:
                                            Title:


                                        CINERGY ENERGY SOLUTIONS, INC.:

                                        By: ____________________________________
                                            Name:
                                            Title:


                                        U.S. ENERGY SYSTEMS, INC.:

                                        By: ____________________________________
                                            Name:
                                            Title:


                                        US ENERGY BIOGAS CORP:

                                        By: ____________________________________
                                        Name:
                                        Title:


                                        ESCROW AGENT:

                                        TANNENBAUM HELPERN SYRACUSE &
                                        HIRSCHTRITT LLP

                                        By: ____________________________________
                                               A Member of the Firm


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