BAGHDAD — Gunmen blew up two Sunni mosques today south of Baghdad, causing heavy damage but no casualties, police said, in an apparent retaliatory attack a day after a suicide truck bombing devastated a revered Shiite mosque in the heart of the capital, killing at least 87 people.

Thousands of U.S. and Iraqi troops, meanwhile, pressed forward on the second day of an operation aimed at clearing out a Sunni insurgent stronghold northeast of Baghdad. The U.S. military said at least 30 al-Qaida fighters were killed and several bombs and weapons caches destroyed as the soldiers fought their way through the streets of Baqouba.

The U.S. military operation that involves some 10,000 American soldiers in Diyala province, an al-Qaida bastion to the north and east of Baghdad, matched in size the force that American generals sent against the insurgent-held city of Fallujah 2Z\x years ago. By late Tuesday, the military had reported only one American death, a Task Force Lightning soldier killed by an explosion near his vehicle.

Iraqi forces also have joined the battle in Diyala. Iraqi Defense Ministry spokesman Mohammed al-Askari said about 5,000 Iraqi soldiers and 2,000 paramilitary police were fighting, while the military said about 1,000 Iraqi soldiers and an equal number of police were involved. The differing numbers could not be reconciled.

The Iraqi Defense Ministry said three civilians had been wounded in Diyala in addition to the 30 al-Qaida militants. It also said 13 suspected al-Qaida fighters had been detained and 14 roadside bombs dismantled, along with three car bombs and three weapons caches.

"The citizens received the valiant Iraqi army forces with overwhelming joy as the soldiers were waving to them with the V for victory sign," the ministry said in a statement.

The head of a Sunni insurgent group that has turned against al-Qaida and is cooperating with U.S. and Iraqi forces in the area said his fighters were participating in the operations and had succeeded in clearing several neighborhoods in eastern and western Baqouba.

The militant leader, who declined to be identified for fear of retribution, spoke as his fighters linked arms, chanted and danced while women ululated in celebration. An Associated Press reporter also saw residents in the Mustafa area in western Baqouba serving food to fighters who had battled al-Qaida and starting to repair their stores.