Baghdad ? Dozens of fighters ambushed a U.S. patrol in Baghdad’s main Shiite militia stronghold Tuesday, firing rocket-propelled grenades and machine gun bursts as the American push into Sadr City increasingly faces pockets of close urban combat.

U.S. forces struck back with 200-pound guided rockets that devastated at least three buildings in the densely packed district that serves as the Baghdad base for the powerful Mahdi Army militia.

The U.S. military said 28 militiamen were killed as the U.S. patrol pulled back. Local hospital officials said dozens of civilians were killed or wounded.

Such street battles – in tight confines and amid frightened civilians – are increasingly becoming a hallmark of the drive into Sadr City and recall the type of head-on clashes last seen in large numbers during last year’s U.S. troop buildup in Baghdad and surrounding areas.