WASHINGTON -- All six crew members aboard a US KC-135 refueling aircraft that went down in western Iraq on Thursday were confirmed dead, the U.S. Central Command said Friday.

The aircraft was lost while flying over friendly airspace on Thursday during Operation Epic Fury, the command said on X.

The command added that the circumstances of the incident are under investigation, claiming "the loss of the aircraft was not due to hostile fire or friendly fire."

The Islamic Resistance in Iraq, an umbrella group for Iraq's pro-Iranian militias, said Friday that its fighters targeted two US KC-135 refueling aircraft over western Iraq.

In two statements, the group said its fighters shot down one KC-135 "with appropriate weapons," and a second aircraft of the same type was also struck before its crew made an emergency landing at "one of the enemy airports."

The KC-135 aircraft, designed to refuel other military planes in flight, has served as the US Air Force's key aerial refueling platform for over 60 years.