Some 4,000 U.S. troops are searching for the missing soldiers in palm groves, marshes and villages near the town of Mahmoudiya, south of Baghdad.
A patrol of 7 U.S. soldiers and their Iraqi army interpreter came under attack Saturday morning outside the Sunni Arab town. The military says four of the soldiers and the interpreter were killed in the attack. The other three are missing.
In an Internet statement published Sunday, the al Qaida-led militant group "The Islamic State in Iraq" claimed it captured several American soldiers during an attack Saturday south of Baghdad. The claim has not been confirmed.
Meanwhile, U.S. Major General William Caldwell says three-thousand more U.S. troops have been sent to Diyala province, northeast of Baghdad, which has grown increasingly violent in recent months.
"If I could add on the Diyala piece, there is a recognition clearly that up in Diyala there's been an uptick in the violence there, due to that fact, General Odinero made the decision to move an additional 3,000 US forces up there over the last six weeks."
Last week, the top U.S. commander in northern Iraq, Major General Benjamin Mixon told reporters that Diyala's government is virtually nonfunctioning and he needs more troops to secure the province.
West of Diyala, in the Kurdish and Arab town of Makhmur, a suicide car bomb blast detonated outside the local offices of a Kurdish political party. The police commander in Makhmur was in his office nearby when the bomb detonated.
The commander says when he came outside he saw the wreckage of the attacker's vehicle which was loaded with iron pipes. He says police are searching for victims buried under rubble.
The blast is the second attack in recent days on Kurdish political and government institutions in the largely peaceful Kurdish north.
Last week, a suicide car bomb attack on government buildings in the capital of the Kurdish autonomous region killed 15 people and wounded some 100 others. The attack was the first major bombing in the city in two years.
Sunni militant groups accuse the Kurdish government and its security forces of collaborating with Americans. There is also widespread tension in parts of northern Iraq over a referendum scheduled for later this year.
The vote on "Article 140" will decide whether several Kurdish and Arab areas will be under the authority of Baghdad's central government or the autonomous Kurdish government.