March 13 (UPI) — The U.S. military confirmed Friday that four of six crew members of a refueling jet on combat operations in the Iran war were killed when it went down over western Iraq in an incident with another U.S. military aircraft.

A rescue operation mounted following the crash on Thursday night was ongoing. The second aircraft landed safely following the incident, which involved neither enemy or friendly fire, U.S. Central Command said in a news release.

“The circumstances of the incident are under investigation. The incident occurred in friendly airspace during Operation Epic Fury, and rescue efforts are ongoing. More information will be made available as the situation develops,” CENTCOM said

The identities of the service members were being withheld until 24 hours after their next of kin had been notified, it added.

CBS News said that the second aircraft, also a Boeing Stratotanker, declared an emergency before landing in Tel Aviv.

The BBC reported that there were six crew on board — a pilot, co-pilot, a boom operator responsible for operating the refueling arm and three others.

An Iraqi intelligence source told CBS the aircraft crashed on the border with Jordan, near the town of Turaibil.

The Iranian military claimed responsibility, saying that an allied militia group in Iraq had downed the aircraft with a missile.

Thursday’s crash came 10 days after three U.S. F-15E Strike Eagles crashed in Kuwait in a friendly-fire incident in which Kuwaiti air defenses “mistakenly shot down” the fighter jets. All six aircrew were rescued after safely ejecting.

The U.S. military’s Stratotanker fleet is a critical asset in its in-flight refueling capability, enabling aircraft to remain airborne for extended periods during missions without having to land to take on more fuel.

The crash in Iraq brings to 11 the number of U.S. military personnel killed since the United States and Israel launched their airborne offensive against Iran on Feb. 28.