A US KC-135 aerial refuelling aircraft crashed in western Iraq killing six crew members, the military said Friday, adding that the incident was not caused by “hostile fire.”

A second plane involved in the incident, which the military said occurred at 2:00 pm Eastern Time (1900 GMT) on Thursday, landed safely.

“Four of six crew members on board the aircraft have been confirmed deceased as rescue efforts continue,” US Central Command (CENTCOM), which is responsible for American forces in the Middle East, said in a statement on X.

An investigation was underway into the crash, the command said, adding that “the loss of the aircraft was not due to hostile fire or friendly fire.”

The deaths bring the number of US service members killed in the ongoing conflict with Iran to at least 11.

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

The Islamic Resistance in Iraq, which is a loose alliance of Iran-backed Iraqi factions, claimed to have downed a KC-135. They also said they had targeted another plane that escaped.

Since the start of the Middle East war, the alliance has been claiming daily attacks on US interests in Iraq and across the region, but it rarely names its targets.

The KC-135 is at least the fourth US military aircraft lost during the war, 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 crew members 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.