A helicopter accident at an air base in the northern Iraqi city of Mosul (search) killed two U.S. soldiers and injured four, the military said Friday.

The crash occurred when an AH-64 Apache (search) helicopter hit a UH-60 Black Hawk that was on the ground, U.S. military spokesman Lt. Col. Paul Hastings said. The accident is under investigation and the cause had not been determined, he said.

The four injured soldiers have returned to duty. They and the two soldiers who died were all from Task Force Olympia, whose headquarters are in Mosul.

The names of the soldiers who died were being withheld until their families are notified.

The Apache is a heavily armed gunship with two crewmembers. The Black Hawk (search) is a transport helicopter that has two pilots and can carry up to 20 passengers.