All six crew members aboard a U.S. refueling aircraft that ​crashed in western Iraq have been killed, the U.S. ‌military confirmed.

"The circumstances of the incident are under investigation. However, the loss of the aircraft was not due to hostile fire or friendly fire," read a statement from U.S. Central Command Friday morning. 

The aircraft was "lost while flying over friendly airspace on March 12 during Operation Epic Fury."

The identities of the service members are being withheld until 24 hours after next of kin have been notified.

The downed aircraft—a KC-135—carries a minimum crew of three including a pilot, co-pilot and a crewmember, known as a boom operator, stationed at the rear of the plane who is responsible for refueling other planes inflight. The task is complex and in high demand as the U.S. Air Force carries out strikes on scores of targets across Iran.

A second aircraft—also understood to be a KC-135—was involved in the crash, but landed safely.

Defense Secretary Pete Hegseth referred to the incident as “tragic” during a Friday morning press briefing at the Pentagon alongside Chairman of the Joint Chiefs of Staff, General Dan Caine.

“War is hell. War is chaos. And as we saw yesterday, with the tragic crash of our KC-135 tanker, bad things can happen. American heroes, all of them,” he said, seemingly referring to the service members aboard the aircraft.

Caine also addressed the “tragic loss” and said the “incident occurred over friendly territory in western Iraq while the crew was on a combat mission.”

The briefing was held when just four of the service members had been announced dead, prior to the remaining two personnel being confirmed as also deceased.

Thursday’s crash brings the total number of American service members killed in the war so far to 13, with around 140 also injured.

Addressing reporters at the Pentagon, Caine paid tribute to the service members who “make an incredible sacrifice to go forward and do the things that the nation asks of them.”