BAGHDAD — A front group for Al Qaeda announced yesterday that it had captured American soldiers in a deadly attack the day before, as thousands of U.S. troops searched insurgent areas south of Baghdad for their three missing comrades.

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 American soldiers and an Iraqi translator.