* U.S. special forces units working with Iraqi troops in Basra

* U.S. military says raid killed 22 suspected militants

* Baghdad under indefinite curfew

By Peter Graff

BAGHDAD, March 30 (Reuters) - The United States confirmed on Sunday that U.S. special forces units were operating alongside Iraqi government troops in Basra, where the government is battling militants loyal to Shi'ite cleric Moqtada al-Sadr.

A U.S. military statement described a joint raid by Iraqi and U.S. special forces units which killed 22 suspected militants, including "16 criminal fighters" strafed in an air strike on three houses.

The raid showed U.S. forces are being drawn deeper into the Iraqi-led crackdown, launched on Tuesday by Prime Minister Nuri al-Maliki in Basra, Iraq's second-biggest city.

The Iraqi special forces team killed four suspected militants in a house and two on a roof before calling in the air strike, the statement said.

"While engaged with hostile forces, ISOF and a supporting U.S. Special Forces team identified additional armed criminal elements in the area," it said. "A supporting Coalition forces aircraft identified enemy forces on three rooftops and engaged with precision gunfire after being cleared by ground forces."

The description of the incident matches a strike the aftermath of which was photographed on Saturday by Reuters. Blood flowed into a sewer from a house. Neighbours had collected a plastic tub of shredded human remains.

Maliki has ordered militants to lay down their arms, extending a 72-hour deadline until April 8. But on Saturday Sadr's aides said the cleric had told his followers not to turn over any weapons to a government that was unable to expel the "occupiers", referring to U.S. forces.

"DEFINING MOMENT"

Despite the six-day operation, Sadr's Mehdi Army fighters remain in control of many Basra streets, manning checkpoints and openly brandishing rifles, machineguns and rocket launchers.

Washington has so far strongly backed Maliki's decision to launch the crackdown. President George W. Bush has called it a "defining moment in the history of a free Iraq".

But the spread of violence risks undoing a year of security improvements and jeopardising plans for U.S. troops to withdraw.

The fighting has established a new main adversary for U.S. forces, whose main military effort for the past year had been against Sunni Arab groups such as al Qaeda.

Sadr is a Shi'ite like Maliki, and helped install the U.S.-backed government in 2006. Although hostile to the United States, he declared a ceasefire last year that U.S. officials said curbed violence. That truce now appears to be in tatters.

On Saturday Maliki said his new foes were "worse than al Qaeda", the Shi'ite-led government's fiercest enemy.

U.S. forces are scarce in the southern Shi'ite areas. British forces pulled out of Basra in December and the remaining British contingent of 4,100 troops has remained on a base outside the city during the latest fighting.

The fighting in Basra has spread to other towns throughout the Shi'ite south of the country and across Baghdad, especially the vast Sadr City slum, named for the cleric's slain father, where his followers have their main power base.

In Baghdad, U.S. forces have been involved in their heaviest fighting in the capital in months, with gunbattles and air strikes that they say have killed scores of fighters.

The capital has been placed under a curfew, which on Saturday night was extended indefinitely. All shops, businesses and schools are closed. (Editing by Catherine Evans)

AlertNet news is provided by