KIRKUK, Iraq (Reuters) - Three car bombs aimed at Iraqi security forces killed at least seven people and wounded 78 in the northern city of Kirkuk on Wednesday, police and hospital sources said. The explosions were the latest in a series of attacks on police and soldiers by insurgents as US troops prepare to withdraw by the end of this year. Kirkuk sits on some of Iraqs biggest oil reserves and is one of the disputed territories at the centre of tensions between the Kurdistan Regional Govt and Iraqs central govt in Baghdad. A police source said the first blast wounded an Iraqi police official, while the second was aimed at a police patrol. The third was outside a building used by Kurdish security forces. Three car bombs exploded in quick succession. We are on high alert and fear there may be more car bombs, the source said. Weve sealed the areas around where the explosions occurred, we are evacuating the wounded and we have intensified searches at checkpoints in other areas. Police and hospital sources said seven people including three policemen were killed and 78 wounded in the blasts in a residential area of southern Kirkuk.