Elsewhere in Baghdad, armed men pulled off a daring armored car robbery, killing two guards and escaping with $850,000, and a suicide car bomber slammed into a convoy carrying Interior Ministry commandos, killing seven of them and two civilians.
South of the capital, two separate bicycle bombings in town markets killed at least seven people and wounded dozens.
The ominous resurgence of violence in the poor Sadr City region began about 1:30 a.m. when an Iraqi patrol searching for three insurgents came under attack. U.S. forces in the neighborhood joined the battle and reported killing between five and eight of the attackers. Iraqi police said eight were killed.
''I am concerned about the events early this morning, but I do not believe this action reflects a pattern of change leading to more violence,'' said Col. Joseph DiSalvo, commander of U.S. forces in east Baghdad.
Al-Sadr's militia, the al-Mahdi Army, was a repeated problem for American forces until a truce was negotiated about a year ago that allowed some U.S. troops to pull out of Sadr City to join the November assault on the insurgent stronghold of Fallujah, west of the capital.
Before the truce, al-Sadr's forces had led unsuccessful but bloody uprisings against coalition forces in Kut and the holy cities of Najaf and Karbala, all south of Baghdad.
With a referendum on Iraq's new constitution less than three weeks away, violence in the poor Shiite district could deepen opposition among al-Sadr's supporters who are bucking mainstream Shiite support for the constitution.
Shiite unity has been seen as critical for passage of the basic law, which minority Sunni Muslims by and large oppose.
A statement read to reporters by an official with al-Sadr's office, accused U.S. forces of trying to draw them into a battle ''aimed at destroying Iraqi towns, particularly those in pro-Sadr areas and . . . to prevent al-Sadr followers from voting'' in the referendum.
The two bicycle bombs hit marketplaces in Hillah, about 60 miles south of Baghdad, where one person was killed and 48 wounded. The second, more deadly bomb went off in Musayyib, about 40 miles south of Baghdad. It killed at least six and wounded 17, including the city police chief, according to police Capt. Muthanna Khalid Ali.
Police also reported finding at least seven bodies in four separate locations in Baghdad - six men who had been bound and shot, including one identified as a policeman, and a woman in her 20s who appeared to have been strangled and tortured.