A U.S. Air Force KC-135 Stratotanker crashed in western Iraq on Thursday during Operation Epic Fury, involving a second tanker but “not due to hostile or friendly fire,” the U.S. military said.

At least five crew members were aboard, and rescue efforts are ongoing, though it is unclear if anyone was injured or killed.

The KC-135, a refueling aircraft also capable of carrying cargo or patients, is one of the oldest Air Force platforms, based on the Boeing 707, with 376 units still active.

The crash marks the fourth known U.S. aircraft loss in the conflict with Iran, following last week’s friendly fire shootdown of three F-15E jets over Kuwait, CNN has reported.

More details are expected as authorities continue rescue operations and gather information for the families of service members.