Four of six U.S. crew members have been confirmed dead after their military refueling plane crashed in Iraq on Thursday while taking part in Iran war operations, the U.S. military has said.

Central Command said in a post on X early Friday that the KC-135 plane went down at approximately 2 p.m. ET in western Iraq, with four crew members confirmed dead. The statement didn’t note the status of the other two crew members but said that “rescue efforts continue.”

“The circumstances of the incident are under investigation. However, the loss of the aircraft was not due to hostile fire or friendly fire,” CENTCOM said.

It said earlier that two aircraft were involved in the incident and that one landed safely.

As is standard protocol in military deaths, the identities of those killed are being withheld until 24 hours after their families have been informed.

The crash means that a total of 10 active service personnel have been killed since Operation Epic Fury began on Feb. 28, when U.S. forces joined with Israel to target senior leaders and military sites in Iran. An 11th died of a medical issue.

Last week, Kuwait mistakenly shot down three U.S. fighter jets but none of the crew were killed.

This is a developing story — check back here for updates soon.