The attack occurred at Adhaim, on the main road between Baghdad and the northern city of Kirkuk.
Police said the insurgents used rocket-propelled grenades and automatic weapons in the attack.
The attack came as U.S. Defense Secretary Donald Rumsfeld said the United States would reduce the number of combat troops in Iraq by two brigades.
Rumsfeld made the announcement while speaking to U.S. soldiers in the Iraqi city of Al-Fallujah today.
"President [George W.] Bush has authorized an adjustment in U.S. combat brigades from 17 to 15,"Rumsfeld said. "The size and composition of U.S. forces of course will fluctuate as commanders continue to shift their focus to training Iraqi security forces."
Rumsfeld did not reveal the exact size of the U.S. troop reduction. But reports say the cut is expected to be between 7,000 and 9,000 soldiers.
Meanwhile, UN spokesman Robert Sullivan said in New York that the UN will not conduct an independent review of the results from last week's Iraqi parliamentary elections.
Sunni opposition groups were calling for an independent review by the UN, alleging there were numerous violations.