Aljazeera reported the bomb exploded in front of Iraqi police headquarters in Tikrit city, which
lies 175km north of Baghdad.
"At approximately 9.30am (0630 GMT) on 11 January in northern Tikrit, anti-Iraqi forces detonated a vehicle-borne improvised explosive device in the vicinity of a police station, killing six Iraqi police," the US military said in a statement.
Aljazeera also reported that a car bomb exploded in al-Dhubat neighbourhood in central Samarra on Tuesday as a joint patrol of US forces and Iraqi National Guard members passed through the area.
No casualties figures have been revealed as yet.
Clashes have also erupted between fighters and US forces in the northern part of Samarra.
With respect to the Tikrit attack, Iraqi journalist Muhammad al-Badri told Aljazeera the booby-trapped car was parked at the side of the road leading to the police headquarters.
There was a loud explosion that rocked the city and columns of smoke were seen rising from the site.
Violence has surged in the run-up
to elections slated for 30 January
Alaa al-Juburi, an Iraqi police officer, told Aljazeera: "The car was parked one kilometre away from the headquarters. The explosion site has been immediately cordoned off and the injured transferred to the hospital.
"The car's number plate has been removed, in order to identify its owners and the attackers behind the explosion."
When asked if a bomber blew himself up inside the car, al-Juburi said: "It seems the explosion has been caused by a wireless time-bomb, not a suicide operation."
Al-Badri said security had been tightened, with Iraqi police setting up checkpoints on several roads in response to warnings of car bombs targeting police headquarters, the governorate building and security forces.
Shortly after the blast, armed men attacked a minibus south of Baghdad, three Iraqi civilians were killed and three wounded in a roadside bombing near Yusufiyah, witnesses and a hospital source said.
"As the Iraqi police continue to get
stronger, and continue
to pose a threat. They will be targeted"
Major Neal O'Brien,
US military spokesman
The bombing was targeting a US
military convoy, but the casualties were on board a passing minibus.
Elsewhere, an Iraqi soldier and a civilian were killed in an attack on an army patrol in the town of Samarra, 125km north of Baghdad, police said.
The attacks come a day after Baghdad's deputy police chief, Amir Nayif, was assassinated, in one of a string of deadly attacks against Iraqi security forces in the run-up to the planned 30 January elections.
"As the Iraqi police continue to get stronger, and continue to pose a threat to the insurgents and terrorists, they will be targeted," US military spokesman Major Neal O'Brien said.
British Prime Minister Tony Blair, the top US ally on Iraq, said on Sunday that the US and Britain would send a team to Iraq to reassess the security situation and to help build up the country's own security forces.