The death toll of clashes between U.S.-backed Iraqi security forces and Shiite militiamen in Baghdad 's eastern neighborhood of Sadr City rose to 22 and 55 others wounded, an interior ministry source said on Sunday.

"Our latest reports said that 22 people were killed and 55 others wounded in the clashes in Sadr City," the source told Xinhua on condition of anonymity.

Earlier, fierce clashes started around midnight after the U.S. troops launched a search for wanted insurgents and weapons in the Mahdi Army stronghold in Sadr City, a police source said.

An Iraqi Army Humvee was destroyed early in the morning by a roadside bomb attack near a military patrol in the 55th Square at the entrance of Sadr City in eastern capital, the source said.

The blast sparked fierce clashes between the troops and Mahdi Army militia, loyal to radical cleric Muqtada al-Sadr, but the source failed to give casualties.

A U.S. military spokesman said a U.S. helicopter in the morning pounded a suspected insurgents' position in the neighborhood, killing nine suspects.

"A U.S. helicopter shelled an insurgent position in Sadr City at about 8:00 a.m. (0500 GMT) and killed nine criminals," Abdul- Latif Raiyan, U.S. spokesman of the military media desk, told Xinhua.

In a separate incident, several mortar rounds landed on a police headquarters in Sadr City, wounding two policemen and damaging several police vehicles.

Iraqi authorities have kept tight security on the Mahdi Army stronghold of Sadr City since Sadr's militiamen fought fierce battles against U.S.-backed Iraqi security forces since March 25.

(Xinhua News Agency April 7, 2008)