* Mehdi Army militia attacks police in Baghdad

* U.S. forces kill six gunmen in ground, air operation

(Adds deaths of two U.S. soldiers in paragraphs four and five)

By Ross Colvin and Randy Fabi

BAGHDAD, March 21 (Reuters) - Mehdi Army fighters attacked police patrols in southern Baghdad, police said on Friday, further fraying a seven-month-old ceasefire called by Shi'ite cleric Moqtada al-Sadr to rein in his militia.

The clashes in Baghdad's Shurta district, which started late on Thursday and continued into Friday morning, follow outbreaks of violence in the southern Iraqi city of Kut in which Mehdi Army fighters battled U.S. and Iraqi security forces.

The fighting took place on the same day that Iraq marked five years since the U.S.-led invasion to topple Saddam Hussein.

A U.S. soldier died from wounds sustained on Friday from "indirect fire", a term usually used to describe rocket or mortar fire, south of Baghdad the U.S. military said. Four other soldiers were also wounded.

Separately, a U.S. soldier died earlier this week when his vehicle overturned in Mandali, a town about 100 km (60 miles) northeast of Baghdad, the U.S. military said.

The deaths bring the number of U.S. soldiers killed in Iraq since the invasion to at least 3,993 according to Web site www.icasualties.org. War critics are likely to seize on the grim milestone of 4,000 to renew calls for a U.S. troop withdrawal.

On Friday, five people were wounded in clashes between Mehdi Army fighters and U.S. and Iraqi security forces in the Hay al-Amil area of southwestern Baghdad, police said.

Sadr, whose militia fought two uprisings against U.S. forces in 2004, first called a ceasefire last August and extended it last month. But two weeks ago he issued a statement telling his followers they could defend themselves if attacked.

Shortly afterwards, gunbattles broke out between Mehdi Army fighters and Iraqi and U.S. security forces in Kut, raising fears that the ceasefire was unravelling. Until Thursday, violence involving the Mehdi Army had been confined to Kut.

U.S. military spokesman Major Mark Cheadle said U.S. forces had launched an operation on Thursday in the western Rashid district of Baghdad, which includes Shurta, to target gunmen firing mortar bombs on civilian areas.

Six gunmen were killed by ground troops and air strikes by Apache attack helicopters and two detained, he said. An Iraqi police lieutenant meanwhile had been kidnapped, he added.

BODIES IN BLACK SACKS

Two police officials, who spoke on condition of anonymity, said the clashes in Shurta began on Thursday night when Mehdi Army fighters launched simultaneous attacks on police patrols and attacked a checkpoint.

"They captured 17 policemen and forced them to take off their clothes. Then they freed them in their underwear," said one of the officials.

The second police source said the gunmen had burnt several cars and captured a number of weapons from the police. U.S. forces later brought eight bodies in black plastic sacks to al-Bayaa police station in southern Baghdad, he said.

U.S. commanders have said Sadr's ceasefire has contributed to a 60 percent drop in violence since last June, allowing them to push ahead with plans to begin withdrawing 20,000 troops sent to Iraq last year to help curb sectarian bloodletting.

They are therefore likely to be worried by any sign that Sadr's truce is no longer being observed by some elements of his militia, who are estimated to number in the thousands. Their main stronghold is in the Baghdad slum of Sadr City.

While Mehdi Army leaders have distanced themselves from the fighting in Kut, rank and file militia members are known to be deeply unhappy with the ceasefire, accusing rival Shi'ite factions and the U.S. forces of using it to attack them.

The fighting comes amid an upsurge in violence in Iraq since January, which the U.S. military has blamed on al Qaeda.

U.S. commanders say the spike does not represent a trend, but analysts warn that al Qaeda and Mehdi Army elements may step up attacks ahead of the November U.S. presidential election. (Writing by Ross Colvin; editing by Philippa Fletcher)

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