A US Air Force KC-135 Stratotanker, a refueling aircraft, went down in western Iraq on Thursday, the US military said, adding the incident was “not due to hostile fire or friendly fire.”

The statement did not specify whether any US service members were hurt or killed.

“The incident occurred in friendly airspace during Operation Epic Fury, and rescue efforts are ongoing,” a release from US Central Command said, using the name the Pentagon has given to US operations against Iran. “Two aircraft were involved in the incident. One of the aircraft went down in western Iraq, and the second landed safely. This was not due to hostile fire or friendly fire.”

At least five crew members were aboard the tanker that went down, a US official told CNN. The second aircraft that landed safely was also a KC-135, the official said.