BAGHDAD (AP) — Two U.S. service members were killed by “enemy forces” while advising and accompanying Iraqi security forces, the U.S. military said in a statement Monday.

The military said in a statement the two were killed during a mission to eliminate a stronghold for the Islamic State group in a mountainous area of north-central Iraq on Sunday.

The names of the service members were withheld pending next of kin notification, in accordance with U.S. Department of Defense policy, the statement said.

A senior Iraqi official, speaking from Baghdad, said a joint mission between Iraq’s counter-terrorism forces and U.S.-led coalition forces south of Makhmour in northern Iraq came under attack from IS militants. During the fighting, two U.S. service members were killed. The official, speaking on condition of anonymity because he was not authorized to speak to reporters, said there was also air support from the U.S.-led coalition.

No further details were immediately available from the U.S. military. The U.S. maintains some 5,200 troops in Iraq on a mission to assist and provide training to Iraqi security forces in the fight against the Islamic State group.