Car bomb at Tikrit police station

Police in Tikrit said the bomber drove into the car park of the police station and blew up his vehicle.

Despite a relentless militant campaign elsewhere, Tikrit has been relatively quiet over the past few months.

In further violence, two policemen were killed in Kirkuk, and two US soldiers were killed north of Baghdad.

Two policemen in Kirkuk were also injured in the attack, which apparently targeted the local police chief.

Positions bombed

The bomb in Tikrit set more than a dozen cars on fire, and charred bodies could be seen in the street, reports said.

US troops sealed off the area immediately after the explosion and set up checkpoints across the city, according to the Associated Press news agency.

Witnesses said the bomber had dressed in police uniform in order to gain access to the police station compound.

The casualty toll was high because the blast occurred at one of the station's busiest times, as dozens of officers arrived to relieve colleagues who had been working overnight, police said.

"He waited until the shift change, then he exploded the car," police chief Saad Daham told the AP news agency.

The attack comes as the Americans continue a big operation against insurgents in the western province of al-Anbar.

The US military says its aircraft have dropped two large bombs on what it called insurgent fighting positions.

There is no word on casualties but the Americans say more than 80 suspects have been detained since the operation they have dubbed Operation River Blitz began on Sunday.