The deaths came from two events, the first a three-hour street fight between the Mehdi militia and U.S. and Iraqi forces, Interior Ministry officials told CNN. Eight people were killed and at least 10 others were injured. There was no immediate report of U.S. casualties.

Soon after, a U.S. airstrike targeted another militia cell in Sadr City with alleged links to Iran, military officials said. At least 32 people were killed and 12 others were arrested, the report said.

The Mehdi brigade is led by popular Shiite cleric Moqtada Sadr, who also leads a political party that is part of the Iraqi government coalition. His whereabouts have been unknown since February when extra U.S. troops flooded Baghdad in security sweeps.

Meanwhile, in a bid to thwart suicide car bombers, the Iraqi government imposed a ban on vehicles in Baghdad as a Shiite pilgrimage to a holy site in northwestern Baghdad began, the report said. The ban on all but official vehicles runs through Saturday.