BAGHDAD, Iraq — A series of car bombs struck the oil-rich northern city of Kirkuk in a two-hour span Saturday, killing at least two people and wounding 30, police said.

The explosions struck various parts of the ethnically mixed city, which has seen an increase in violence even as Iraqi and U.S. forces gear up for a sweep to pacify violence-ridden Baghdad.

Violence also erupted elsewhere in cities to the north of the capital, with clashes in Mosul prompting the city to impose a vehicle ban while four policemen were killed in an attack on a checkpoint in Samarra, police said.

The attacks in Kirkuk started about 9:40 a.m. when a car packed with explosives blew up near the offices of the Kurdish Democratic Party of Massoud Barzani, leader of Iraq's autonomous Kurdish region.

At least two people were killed and 30 wounded, including five KDP guards, in that blast in eastern Kirkuk, police Col. Anwar Hassan said.

Another car bomb exploded about 20 minutes later near a girls' school in the south of the city, but the facility was closed for the weekend and no casualties were reported, Hassan said.

A third car bomb hit a gas station at 10:10 a.m. in Kirkuk, wounding three civilians, followed by two other parked-car bombs in a commercial area about 20 minutes after that in the southern half of the city.

A sixth car bomb exploded elsewhere in southern Kirkuk at 11:50 a.m., while two roadside bombs targeted police patrols at about the same time in a predominantly Christian area in the north of the city.