"One of the aircraft went down in western Iraq, and the second landed safely. This was not due to hostile fire or friendly fire," US Central Command (CENTCOM), which is responsible for American forces in the Middle East, said in a statement.

Iran's military however claimed in a statement carried by state TV that an allied group in Iraq had downed the aircraft with a missile, killing all its crew.

The KC-135 is at least the fourth US military aircraft lost during the war in the Middle East, after three F-15s were shot down by friendly fire over Kuwait.

KC-135s, which have been in operation for more than 60 years, generally have a crew of three -- a pilot, a copilot and a third who operates the boom used to refuel other aircraft, according to the US Air Force.

But some KC-135 missions require a navigator, and the aircraft can carry up to 37 passengers, an Air Force factsheet said.

Early in the war -- which began on February 28 -- Kuwaiti forces mistakenly downed three American F-15E fighters, but all six crewmembers were able to eject, according to CENTCOM.

That incident occurred during combat including "attacks from Iranian aircraft, ballistic missiles, and drones," the military command said at the time.