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 U.S. troops prepare to withdraw by the end of this year.
Kirkuk, inhabited by a mix of Kurds, Arabs, Turkmen and others, sits on some of Iraq's biggest oil reserves and is one of the disputed territories at the center of tensions between the Kurdistan Regional Government and Iraq's central government 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.
"We've 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, 250 km (155 miles) north of Baghdad. One hospital source said many of the wounded were policemen, but was unable to give an exact figure.
A police official said many cars and houses had been badly damaged and firefighters were trying to put out fires.
Kirkuk province police chief Jamal Tahir said an investigation showed the third explosion, which targeted a Kurdish security building, was carried out by a suicide bomber.
"What we have discovered is that at least one attack was carried out by a suicide truck bomber," Tahir told reporters.
"Militant armed groups had threatened to target security forces in Kirkuk for recent success in pursuing terrorists and bringing them to justice."
The blast also damaged a branch office of Kurdish premier Masoud Barzani's Kurdistan Democratic Party, a police official said.
While violence in Iraq has declined sharply from the height of sectarian warfare in 2006-07, bombings still occur daily, and Sunni insurgents and Shi'ite militia stage lethal attacks.
Nearly 200 people died and hundreds were wounded last month in bomb attacks bearing the hallmarks of Sunni Arab insurgents.
"How long will this bad and unstable situation last? We call on the state and the government of Kirkuk to take tough security measures to prevent such incidents from happening again. The victims are poor people," said Kirkuk resident Ahmed Zebari.
(Writing by Serena Chaudhry; Editing by Janet Lawrence)