Multiple Islamic State suicide bombers attacked a base in northern Iraq where U.S. military advisers are stationed, security sources said on Sunday.

Two militants detonated their vests at the entrance to the K1 base overnight, and the remaining three were killed by Kurdish peshmerga forces who control the Kirkuk area where it is situated.

“They were wearing uniforms like the Kurdish peshmerga and had shaved their beards to look like us,” one officer told Reuters, adding that two peshmerga had been killed in the attack. A further six were wounded.

Iraqi forces backed by a U.S.-led coalition are fighting to dislodge Islamic State from Mosul, 140 km northwest of Kirkuk, but large pockets of territory remain under militant control, including Hawija, which is near the targeted base.