(Adds Baghdad car bomb, paragraph 8, Iraq deputy PM 16-17)

By Ibon Villelabeitia and Mariam Karouny

BAGHDAD, May 13 (Reuters) - Thousands of American troops searched on Sunday for three U.S. soldiers missing in Iraq after an ambush in which al Qaeda said it seized "crusader" forces, while a suicide bomber killed 50 people in the Kurdish north.

The self-styled Islamic State in Iraq, a group led by al Qaeda, said in an Internet posting it was holding soldiers who survived an attack south of Baghdad in which the U.S. military said four U.S. troops and an Iraqi army translator were killed.

That attack and the suicide truck bombing came as President George W. Bush deploys 30,000 more U.S. troops due in Iraq in June in what is seen as a final push to halt a slide into all-out civil war between majority Shi'ites and Sunni Arabs.

Last June, al Qaeda abducted two U.S. soldiers in the same area where the patrol of seven U.S. soldiers and one Iraqi army interpreter were ambushed on Saturday. Their badly mutilated bodies were found days later.

Iran and the United States, making a cautious diplomatic rapprochement, said they would hold talks in Baghdad aimed at stabilising Iraq.

The White House said U.S. and Iranian officials would meet in the next few weeks. The U.S. ambassador to Iraq, Ryan Crocker, will head the U.S. team.

Talks between Shi'ite Iran and the United States, which accuses Tehran of supplying and training Shi'ite militia in Iraq, are rare. The two countries, at loggerheads over Iran's nuclear programme, have not had diplomatic ties for more than a quarter century. Iran denies charges it stirs trouble in Iraq.

In Baghdad, a car bomb killed 12 people in a popular market in a mostly Shi'ite area, police said. The attack was of the kind U.S. and Iraqi officials say are carried out by Sunni al Qaeda in a campaign to stoke sectarian passions heightened since a Shi'ite shrine was bombed in Samarra in 2006.

KURDS FEAR VIOLENCE

As U.S.-led troops backed by helicopters and jets combed the ambush area south of the capital known as the Sunni "Triangle of Death", a truck bomb killed 50 people and wounded 70 in the northern town of Makhmour, the governor said. It was the second attack on Kurdish areas in Iraq in four days.

Makhmour is just outside the autonomous Kurdish region, but Kurds want to include it in the region in a future settlement.

A truck bomb on Wednesday in the city of Arbil, capital of Kurdistan, killed 15 people and wounded more than 100 in an attack claimed by al Qaeda that sparked fears violence engulfing much of Iraq was spreading to the relatively peaceful region.

In a statement, Islamic State in Iraq claimed responsibility for the attack on the U.S. patrol, but gave no proof.

"God has enabled your brothers at the Islamic State in Iraq on Saturday ... to clash with a crusader patrol in Mahmudiya area at the southern part of Baghdad," it said.

"Some were detained and some were killed," it said. "We will provide the full details of this blessed operation as soon as they are available."

Major-General William Caldwell, chief spokesman for the U.S. military in Iraq, told a news conference U.S. troops would make "every effort available to find our three missing soldiers."

Iraqi Deputy Prime Minister Barham Salih told CNN television al Qaeda appeared to have abducted the soldiers, though he was not sure the Web site that carried the group's claim was authentic.

"No one can be complacent about al Qaeda and its affiliate organizations and perhaps one can say because of the pressures on al Qaeda in Baghdad ... they are adapting and moving into other areas in trying to inflict mayhem in those areas," he said of the attacks carried out on Sunday.

Residents said the patrol was ambushed after it struck a roadside bomb on a rural road in an area of palm groves.

"We saw smoke rise from the area. Three vehicles were on fire and a fourth one had fallen into a canal," said a farmer.

Duraid Kashmula, governor of Nineveh province, said the huge blast in Makhmour killed 50 and wounded 70 and was aimed at a government compound that also housed a Kurdish party office.

Other security sources said local officials of the KDP party of Kurdish leader Massoud Barzani were meeting at the time.

The blast left a wide crater and many buildings in ruins. (Additional reporting by Paul Tait in Baghdad, Shamal Aqrawi in Arbil, Parisa Hafezi in Tehran and Tabassum Zakaria in Washington)

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