In the most recent attack, officials say unidentified gunmen fired at a police checkpoint on the road between the towns of Fallujah and Ramadi, west of Baghdad.

Late Wednesday, U.S. troops came under a rocket and mortar attack near the town of Baquba, north of the capital. Two American soldiers were killed and one was seriously wounded. More than 500 U.S. military personnel have died in Iraq since the start of the war. Also Wednesday, unknown assailants attacked a mini-bus near Fallujah, carrying women between their homes and their work at an American military base. Some of the survivors are quoted as saying they may have been targeted because they worked for the Americans.

Brigadier General Mark Kimmitt says this is part of an ongoing shift in strategy by anti-coalition elements.

"They are moving away from attacking, as their primary target, coalition forces and moving on towards attacks against Iraqi civilians or security forces, Iraqi government officials," he said. "We believe that the purpose behind that is to send a message of terror to those people that, if you work for the coalition, that if you work alongside and try to support the coalition, we can reach out and touch you."

Baquba, Ramadi, and Fallujah are in the so-called Sunni triangle, where most of the deadly attacks against coalition forces have taken place.