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BAGHDAD, June 10 (Reuters) - A car bomb exploded on a bridge often used by U.S. forces south of Baghdad on Sunday and part of the bridge collapsed, possibly trapping some U.S. soldiers, police said.

A U.S. military spokeswoman said there were reports of wounded in the incident but no other information was immediately available from the military. The attack took place near al-Iskandariya, 40 km (25 miles) south of Baghdad.

"There may be American troops under the bridge," a Babil province police source said.

On Saturday, at least 12 Iraqi soldiers were killed and 30 wounded when a suicide truck bomber hit an Iraqi army checkpoint on a road between al-Iskandariya and Jurf al-Sakhar near the Babil provincial capital Hilla, a predominantly Shi'ite city.

Unrelenting violence between majority Shi'ites and Sunni Arabs, dominant under Saddam Hussein, has raised fears of all-out sectarian civil war in Iraq.

The U.S. and Iraqi military have poured thousands of extra troops into a security crackdown in Baghdad and other areas in a last-ditch attempt to avert such a war.

Attacks blamed on al Qaeda and other Sunni Islamist groups have been on the rise outside Baghdad since the four-month-old crackdown began, forcing insurgents out of the capital and into outlying cities and towns.

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