"Meanwhile, a U.S. helicopter fired on two checkpoints manned by U.S.-allied Sunni fighters near Samarra, 60 miles north of Baghdad, killing six and injuring two, Iraqi police said. It was the latest in a string of controversial "friendly fire" incidents in which U.S. forces have called in strikes on their new allies. Thousands of Sunnis have abandoned the insurgency and joined pro-U.S. defense groups, known as "awakening councils," a move credited with helping reduce violence in recent months. An Iraqi army commander and one of the paramilitary group's leaders both said the men were staffing a checkpoint when they came under fire."