Five coalition soldiers have been killed and three are missing after an attack by insurgents south of the Iraqi capital Baghdad, the US military says.
A massive search operation has been mounted near Mahmudiya
The patrol of seven Americans and their Iraqi interpreter were attacked near the town of Mahmudiya, spokesman Maj Gen William Caldwell said.
US and Iraqi forces have mounted a massive search operation and roadblocks have been set up around the area.
The US military says it is using thermal equipment to search overnight.
Gen Caldwell said the team was attacked seven miles (11 kilometres) west of Mahmudiya, scene of frequent attacks by insurgents against US troops.
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.
The US military has not clarified if the Iraqi interpreter was among the missing or those killed.
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.
An extra 30,000 US troops are being deployed in Baghdad and Anbar province as part of a four-month old joint Iraqi-American security drive.
US commanders say the "surge" in troops has reduced the number of sectarian murders in the capital but the number of car bombings remains undiminished at more than 100 a month, says the BBC's Andrew North in Baghdad.
More than 3,300 US soldiers have been killed in Iraq since the war began in March 2003.