The patrol of seven Americans and their Iraqi interpreter were attacked near the town of Mahmudiya, spokesman Maj Gen William Caldwell said.

He said that within an hour other troops were at the scene of the attack, hunting for the missing soldiers.

Checkpoints have been set up to prevent the soldiers being moved from the area.

Gen Caldwell said the team was attacked seven miles (11 kilometres) west of Mahmudiya, scene of frequent attacks by insurgents against US troops.

Major operation

Information of the attack first emerged when an unmanned aerial vehicle spotted two burning vehicles just before 0500 (0100 GMT) .

A huge operation has been launched involving helicopters, unmanned aerial vehicles and fixed-wing aircraft, to try to find the missing men.

The US military is also talking to local leaders to try to enlist their help in the search, while checkpoints have been set up on roads leading away from the area and west into Anbar to prevent the soldiers being taken to an insurgent base.

"Make no mistake: we will never stop looking for our soldiers until their status is definitively determined, and we continue to pray for their safe return," said Gen Caldwell.

In the past year, six US soldiers have been abducted and killed by insurgents in two similar incidents.

The bodies of two soldiers were found days after they went missing in the same area last June, while four soldiers were abducted and killed by insurgents in Karbala in January this year.