Four U.S. service members were killed when a KC-135 refueling aircraft crashed in western Iraq Thursday afternoon, officials confirmed.

A U.S. KC-135 refueling aircraft went down in western Iraq around 2 p.m. ET, U.S. Central Command confirmed early Friday. Four of the six crew members aboard the aircraft have been confirmed dead as rescue efforts continue.

The circumstances surrounding the incident remain under investigation. Officials said the loss of the aircraft was not the result of hostile fire or friendly fire.

The identities of the service members are being withheld pending notification of next of kin and will be released 24 hours after those notifications are complete.

This is a developing story. Check back for updates.