Kirkuk – At least 33 people have been killed in a co-ordinated suicide car bombing and gun attack on a police base in northern Iraq.A car bomb was set off before two militants dressed in police uniforms and armed with guns, grenades and suicide vests stormed and sought to take control of the compound in the city of Kirkuk, police said.A further 70 people were also injured in the deadly attacks - shattering a relative calm in recent days in the war-torn country.Police said there were still bodies trapped under the collapsed debris of buildings following the blast.The attackers struck during the morning rush hour in the city centre, said Brigadier General Natah Mohammed Sabr, head of the city's emergency services department.In addition to the casualties, the blast caused extensive damage to nearby buildings and vehicles.The gunmen burst threw the main gates of the police base in the direction of the headquarters block.They threw multiple grenades, but were killed before they could get there, witnesses said."I saw a vehicle stop at the checkpoint at the main entrance, and the police started checking it," said Kosrat Hassan Karim, who was nearby when the attack took place."Suddenly, a loud explosion happened, it was terrifying."I saw many people killed inside their cars - I have never seen such a big explosion in my life."No group immediately claimed responsibility for Sunday's attack.Kirkuk, an ethnically mixed city, is at the centre of a dispute over oil and land rights between Baghdad's central government and the autonomous northern Kurdish region.The unresolved row is persistently cited by diplomats and officials as the biggest threat to Iraq's long-term stability.Tensions between Shiite, Kurdish and Sunni factions in Iraq's power-sharing government have been on the rise this year.Militants continue to strike almost daily, and carry out at least one big attack a month.The latest violence comes amid weeks of protests calling for Prime Minister Nuri al Maliki to resign.Last month a suicide bomber disguised as a mourner killed at least 26 people at a funeral at a Shiite mosque in the nearby city of Tuz Khurmato.Days earlier, a suicide bomber driving a truck killed 25 people in an attack on a political party headquarters in Kirkuk, which is 105 miles north of the capital Baghdad.Agencies