A suicide car bomber has killed at least 18 people in an ambush in the northern Iraqi city of Kirkuk.
Police officers were lured to a market area by a small bombing, then the bomber rammed a patrol car, setting off a larger explosion.
Kirkuk has been the scene of frequent attacks on police by insurgents waging war on US-led multinational forces and their Iraqi allies.
At least 25 people are reported to have been injured in the latest attack.
The bomber struck near the main road leading to Baghdad on the outskirts of Kirkuk, according to reports.
About half those killed were police officers, a police spokesman said. Many of the wounded were also police.
Correspondents say some of Kirkuk's tensions are caused by the mixture of Arab, Kurdish and Turkmen inhabitants, all of whom claim ownership of the city and the oil-rich territory around it.
A mortar was fired as a palace was handed over by the US to Iraq
A week ago four policemen were shot dead in an ambush east of Kirkuk, while three others were killed and three wounded by a roadside bomb in the centre of the city.
The Kirkuk attack takes to at least 160 the number of Iraqis killed in suicide operations since Friday.
It came hours after insurgents fired a mortar round during a US ceremony to transfer one of Saddam Hussein's former palaces in his hometown of Tikrit to Iraqi control.
No-one was injured but TV footage showed guests including the US ambassador to Iraq, Zalmay Khalilzad, and the US commander in Iraq, Gen George Casey, rushing for cover.
The shell failed to detonate properly and the ceremony continued after a brief interruption, the US military said.
Three US soldiers have been killed since Saturday, the US military said, bringing the American death toll in Iraq since the 2003 invasion to at least 2,100.