The statement came on one of the deadliest days in the country in recent weeks, with at least 124 people killed or found dead. A suicide truck bomb tore through the offices of a Kurdish political party in northern Iraq, killing 50 people, and a car bombing in a crowded Baghdad market killed another 17.
Troops surrounded the town of Youssifiyah and told residents over loudspeakers to stay inside, residents said. They then methodically searched the houses, focusing on possible secret chambers under the floors where the soldiers might be hidden, residents said. The soldiers marked each searched house with a white piece of cloth.
Soldiers also searched cars entering and leaving the town, writing "searched" on the side of each vehicle they had inspected. Several people were arrested, witnesses said.
It's a massive search, including 4,000 U.S. troops, reports CBS News correspondent Mark Strassmann. And increasingly desperate, to ease the anguish of the missing and their families.
On its web site, the Islamic State in Iraq claimed credit for the ambush, and for capturing "a number of crusader soldiers."
It said more details — implying proof — will come soon, adds Strassmann.
The Islamic State in Iraq offered no proof for its claim that it was behind the attack in Mahmoudiya that also killed four U.S. soldiers and an Iraqi translator. But the Sunni area known as the "triangle of death" is a longtime al Qaeda stronghold.
If the claim proves true, it would mark one of the most brazen attacks by the umbrella Sunni insurgent group against U.S. forces here.
Maj. Gen. William Caldwell, spokesman for the U.S. military, said U.S. troops backed by aircraft and intelligence units were scouring the farming area as the military made "every effort available to find our missing soldiers."
President Bush was also getting regular updates on the missing soldiers, said Gordon Johndroe, a spokesman for the White House's National Security Council in Washington.
The early morning attack on two U.S. military vehicles outside of Mahmoudiya, about 20 miles south of Baghdad, left the bodies of the four U.S. soldiers and their translator badly burned.
Caldwell said the bodies of the interpreter and three of the slain soldiers had been identified, but the military was still working to identify the fifth.
Later Sunday, the Islamic State of Iraq posted a brief message on a militant Web site saying it was responsible for the attack and held an unspecified number of U.S. soldiers. The group promised more details later.
The Islamic State is a coalition of eight insurgent groups. Late last month, it named a 10-member "Cabinet" complete with a "war minister," an apparent attempt to present the Sunni coalition as an alternative to the U.S.-backed, Shiite-led administration of Prime Minister Nouri al-Maliki.
U.S. military officials said they had no indication of who was behind Saturday's attack.
"It's difficult to verify anything that al Qaeda in Iraq would say because they lie," said Lt. Col. Christopher Garver, a military spokesman. However, "it would not surprise us if it were al Qaeda behind this, because we've seen this type of attack, this type of tactic, before."