BAGHDAD - An al-Qaida front group announced Sunday it had captured American soldiers in a deadly attack the day before, as thousands of US troops searched insurgent areas south of Baghdad for their three missing comrades.

Iraqis and US soldiers survey the scene following a car bomb attack in Baghdad, Iraq, Sunday, May 13, 2007. A parked car bomb exploded near a market in central Baghdad, killing at least 12 Iraqis, wounding 43 and damaging shops, police said. [AP]

The statement came on one of the deadliest days in the country in recent weeks, with at least 126 people killed or found dead - including two American soldiers who died in separate bombings. 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.

The Islamic State in Iraq offered no proof for its claim that it was behind the attack Saturday in Mahmoudiya that also killed four US soldiers and an Iraqi translator. But the Sunni area known as the "triangle of death" is a longtime al-Qaida stronghold.

If the claim proves true, it would mark one of the most brazen attacks by the umbrella Sunni insurgent group against US forces here.

Maj. Gen. William Caldwell, spokesman for the US military, said 4,000 US 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 US military vehicles outside of Mahmoudiya, about 20 miles south of Baghdad, left the bodies of the four US 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 US 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 US-backed, Shiite-led administration of Prime Minister Nouri al-Maliki.

US military officials said they had no indication of who was behind Saturday's attack.

"It's difficult to verify anything that al-Qaida 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-Qaida behind this, because we've seen this type of attack, this type of tactic, before."

Insurgents also launched attacks across the country Sunday, with a suicide bomber in northern Iraq slamming a truck into local offices of the Kurdistan Democratic Party, which is headed by Massoud Barzani, leader of the autonomous Kurdish region in northern Iraq.

Cars were charred and crushed by the blast in Makhmur, a town with a substantial Kurdish population just south of the autonomous Kurdish-controlled areas.

At least 50 people were killed and 115 were wounded, including the city's mayor, Abdul Rahman Delaf, who also is a prominent Kurdish writer, and the director of the KDP office, said Ziryan Othman, the health minister of the Kurdish regional government.

"Makhmur is an open, peaceful area, and al-Qaida is trying to destabilize it by causing fighting between Arabs and Kurds," said Qassim Amin, who son and daughter - who both work for the party - were injured.

The attack was the second suicide bombing in Kurdish areas in five days. On Wednesday, a suicide truck bomber devastated the security headquarters in Irbil - the capital of the Kurdish autonomous region and one of Iraq's most peaceful cities - killing at least 15 people and wounding more than 100. The Islamic State of Iraq also claimed responsibility for that blast.

In Baghdad, meanwhile, a parked car exploded near the popular Sadriyah market in the center of the city Sunday, killing at least 17 people and wounding 46, police said. AP Television News footage showed a crater in the ground filled with debris, splintered wood, metal and a tire.

"I saw pools of blood and charred pieces of flesh," said Firas Fhadil, the owner of a nearby electrical appliances shop.

Market workers used fruit carts to evacuate the casualties, because road closures made it difficult for ambulances to reach the area, he said.

Sadriyah has been hit by several blasts usually blamed on Sunni insurgents, who are suspected of targeting commercial areas to kill large numbers of people and derail the Baghdad security crackdown that began 12 weeks ago.

On April 18, 127 people were killed in a car bombing in the same area - one of four bombings that day that killed a total of 183 people.

The US soldiers who died Sunday were killed in bombings in Anbar and Salahuddin provinces, the military said.

With violence on the rise, Caldwell announced that an additional 3,000 forces have been sent to Diyala province, a province northeast of Baghdad and scene of recent sharp fighting.

Last week, the top US commander in the north, Maj. Gen. Benjamin Mixon, said he didn't have enough troops to restore order in Diyala.

On Sunday, Iraqi gunmen drove into the Diyala capital of Baqouba, pulled two handcuffed men out of the trunk and shot and killed them, police and witnesses said.

"This is the destiny of traitors," the gunmen yelled. Three other civilians also were killed execution-style in a market in the city center, police said.