Date: 202101
Local time of day: 1201-1800
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZZ
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Communication Breakdown
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Aircraft; Ground Incursion Runway
Detector: Person Air Traffic Control; Person Flight Crew
When detected: Other Takeoff Roll
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Rejected Takeoff
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: The incident took place at ZZZZ on runway XX; where aircraft must back taxi to use full length. The CA (Captain) and I had just deadheaded down; with our first flying leg of the trip being from ZZZZ to ZZZ. I was feeling refreshed as I had napped for about 2 hours on the [deadhead] down. We had 1.5 hours on the ground; so did not feel rushed; even though it was my first time to ZZZZ. Boarding; preflight; briefing; clearance; pushback; start and taxi to hold short of Rwy XX were uneventful. The CA briefed language as a threat; but the clearance and ground control went smoothly. Initially we held short at taxiway E. Following a departure by another AC we were cleared to back taxi down to Runway XX approach end; and lineup and wait. While in our turn around; an aircraft checked in on final for our runway; I visually identified that aircraft and said aloud that he looked to be on a 6 mile final. Tower told that aircraft to continue.Once aligned with the centerline; I confirmed with the Captain that he was ready to depart; then told Tower Aircraft X in position; ready for takeoff'. Tower responded that we were cleared for takeoff runway XX; and to contact departure when airborne. I replied that we were cleared for takeoff; runway XX; and echoed the departure frequency we were just given.The Captain and I confirmed with each other we were both ready to go and he pushed up the throttles. As we both were watching the engines spool up; we began to roll forward down the runway. Tower then said something to the effect of "Aircraft X; please do not takeoff; there is an aircraft...' As I started to look up; my first thought was about the aircraft on final. The CA and I both looked at each other; confused about the nonstandard radio call. Tower sounded a bit flustered; and then I saw 2 strobe lights at the far end of the runway. I said 'Reject' aloud; then answered tower "Aircraft X rejecting takeoff.' The CA executed the rejected takeoff; but at only 40 knots or so; was really just bringing throttles to idle stopping the aircraft. I called 'Remain seated; remain seated' on the PA.At that point I started to see an aircraft that went with those strobe lights and a white twin engine civilian airplane lifted off our runway. Somehow this aircraft had gotten onto or been cleared onto RWY XX for an intersection takeoff. There had been Spanish being spoken on the frequency but I did not connect that discussion with an aircraft on our runway. The same frequency is used for clearance; pushback; ground; and tower control.As we stopped...perhaps 500-1000 ft. down the runway; tower then told us we were cleared for Takeoff again. We replied negative that we would need to taxi clear or do a 180 turn and go to the approach end. Tower then told the aircraft on final to go around while we taxied. The CA called for the Rejected takeoff checklist; which we ran. We used the card first then QRH back cover. The CA also used the PA to briefly explain what had just happened. We quickly discussed our options; and decided that we could do a 180; and rerun the before takeoff checklist. Subsequent lineup; and takeoff were uneventful."




Synopsis: B737 First Officer reported rejecting takeoff due to ground conflict with departing aircraft.