Updated

A car bomb exploded on the outskirts of Fallujah (search) on Monday and there were reports of U.S. casualties, a U.S. military official said on condition of anonymity.

Further details were not immediately available.

U.S. forces have not patrolled inside Fallujah since April, when U.S. Marines ended a three-week siege and handed over control of security to the Fallujah Brigade, which is made up of officers who served in Saddam Hussein (search)'s army.

The city has remained a problem spot for American forces, who say Fallujah is a center for insurgent activity.