BAGHDAD -- Three Sunni Muslim mosques were attacked and burned south of Baghdad on Thursday, Iraqi police said, in apparent reprisal attacks after suspected al-Qaida militants blew up the minarets of a revered Shiite shrine.

Thousands of Iraqi and U.S. soldiers were on the streets of Baghdad and other cities enforcing curfews imposed after Wednesday's bombing that toppled the two golden minarets of Samarra's al-Askari mosque.

An attack on the same mosque in February 2006 unleashed waves of sectarian violence in which tens of thousands of people were killed, tipping Iraq close to all-out civil war between majority Shiite Muslims and minority Sunni Arabs.

The latest Samarra attack, condemned as barbarous by U.S. President George W. Bush, immediately raised fears of similar retaliatory violence, but the reaction has so far not matched the surge in sectarian killings after the 2006 attack.

Prime Minister Nouri al-Maliki has ordered an extra brigade of Iraqi security forces to Samarra, the U.S. military said. "We want to make sure we don't see a repeat of what happened after February 2006," U.S. military spokesman Christopher Garver said.

The streets of Samarra were quiet Thursday, but there was a grim mood as Iraqi soldiers fanned out around the mainly Sunni city. Residents said military snipers could be seen on rooftops.