At least 85 people have been killed by a suicide truck bomb in the volatile Iraqi city of Kirkuk. A Reuters cameraman, who witnessed the carnage, said some of the victims were trapped on a bus where they were burned alive. Police also said 180 people were wounded and they warned the death toll could rise from the blast that heightened tension in the northern city, shared by Kurds, Turkmen, Shi'ite and Sunni Arabs. The blast, near an office of the Patriotic Union of Kurdistan of Iraqi President Jalal Talabani, was one of several attacks in Kirkuk, a city facing a crucial referendum this year. It's to vote on whether to join semi-autonomous Kurdistan. Dozens of cars were set on fire and houses and shops destroyed by the force of the blast. Police said 25 of the wounded were in a critical condition and many bodies might still be buried in the rubble. The truck detonated minutes apart from a car bomb in a busy Kirkuk shopping area that wounded two people. A police officer was killed and four officers were wounded soon after, when a parked car bomb exploded in southern Kirkuk, police said. A fourth car bomb was discovered and made safe. South of Baghdad, thousands of U.S. troops swooped on a suspected al Qaeda in Iraq safe haven used to reinforce militants fighting in the capital, the military said. U.S. and Iraqi forces have launched a series of big security clampdowns since the last of 28,000 extra U.S. troops ordered to the country by U.S. President George W. Bush arrived last month. They aim to thwart violence between majority Shi'ites and minority Sunni Arabs which has pushed Iraq towards civil war, while winning time for Shi'ite Prime Minister Nuri al-Maliki to deliver key power-sharing laws.