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By Ibon Villelabeitia

BAGHDAD, May 12 (Reuters) - Seven U.S. soldiers and an Iraqi army interpreter were attacked while on patrol in an al Qaeda stronghold south of Baghdad on Saturday. Five were killed and three are missing, the U.S. military said.

The attack, one of the worst against American ground forces since a U.S.-backed security crackdown began in Baghdad three months ago, took place near the town of Mahmudiya, in the same area where two U.S. soldiers were abducted by al Qaeda insurgents last year before their mutilated bodies were found.

U.S. forces have launched a search operation for the missing soldiers, using helicopters, unmanned drones and jets, and have set up checkpoints in the area.

U.S. spokesman Lieutenant-Colonel Christopher Garver said it was not immediately clear if the Iraqi army translator was among the dead or missing.

"Coalition forces are currently using every means at our disposal to find the missing soldiers, and we will continue these efforts until all are accounted for," Major-General William Caldwell, chief military spokesman for U.S. forces in Iraq, said in a statement.

Residents in Mahmudiya said gunmen wearing Iraqi army uniforms had stopped a U.S. patrol comprised of three Humvees at a fake checkpoint before attacking the soldiers, an Iraqi army source told Reuters.

The area of orchards and palm groves south of Baghdad is a bastion of Sunni Arab militants including al Qaeda. The district is known as the Sunni "Triangle of Death" for the large number of attacks against U.S. and Iraqi forces.

Thirty thousand additional U.S. troops are being deployed in Baghdad in what is regarded as a final push to halt Iraq's slide into all-out civil war between majority Shi'ites and once-dominant Sunni Arabs.

U.S. commanders have said the push has reduced sectarian murders in Baghdad but has driven militants to regroup in the city's outskirts.

Once all five brigades sent by U.S. President George W. Bush as part of a "surge" are in place in June, the military will have more firepower and troops to try to secure the outlying areas from where Sunni Arab militants are staging attacks against Shi'ite targets in the capital, commanders have said.

"TRIANGLE OF DEATH"

Last June, al Qaeda militants abducted two U.S. soldiers in nearby Yusufiya in an attack on a U.S. checkpoint in which a third U.S. soldier was killed. The mutilated and bobby-trapped bodies of the two U.S. soldiers were found days later.

Caldwell said that after the attack at 4:44 am a nearby unit heard explosions, suggesting a coordinated operation.

Minutes later a U.S. drone observed two burning vehicles. When a rapid-reaction force arrived one hour after the attack they saw five soldiers dead, Caldwell said.

The three other soldiers are listed as "duty status and whereabouts unknown", he said.

Bush is facing increasing pressure from Democrats to set timetables for withdrawing American troops in a war that has killed more than 3,300 U.S. soldiers and tens of thousands of Iraqis since the invasion in 2003.

Debate over funding the war in Washington has moved to the Senate after Democrats in the House of Representatives defied Bush on Thursday by passing a bill that allows money to continue combat only for two or three months.

Bush vetoed a $124 billion war funding bill last week because it set a deadline for the withdrawal of 150,000 U.S. troops.

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