The attack, which took place in Anbar province, began with a barrage of rocket fire on a military headquarters in the town of Haditha.
Meanwhile, militants in a convoy of seven vehicles, two of which were car bombs, gunned down six policemen at a checkpoint before heading to a police station inside the town.
The two car bombs were blown up at the gate to the police station, killing three policemen.
No group immediately claimed responsibility for the attack. The predominantly Sunni province of Anbar shares a long border with Syria and is a stronghold for al Qaeda.
Sunni insurgents have been crossing into Syria from Iraq to join ranks with rebels fighting to overthrow President Bashar al Assad, whose Alawite sect derives from Shi'ite Islam.
Al Qaeda's Iraqi wing merged with its Syrian counterpart this year to form the Islamic State of Iraq and Levant, which has claimed responsibility for attacks on both sides of the border.
In a separate incident, a military helicopter crashed and two Iraqi soldiers and three militants were killed in clashes northeast of Baghdad, where Iraqi army units were launching a big operation, military sources said.