Dozens of people have been reported killed when at least one suicide car bomber and gunmen tried to storm police headquarters in the northern Iraqi city of Kirkuk.
There has been no official confirmation yet of the number of people killed or injured in the attack early on February 3.
Health officials said at least 16 people were killed. A senior police officer, Brigadier-General Naseh Muhammad, put the death toll much higher.
"Two suicide car bombs went off in front of Kirkuk police headquarters and Kirkuk telephone exchange building, killing more than 30 people and wounding more than 70," he said. "The Kirkuk exchange building was heavily damaged by the blast. Suicide bombers also attacked Kirkuk police headquarters with hand grenades and Kirkuk police engaged these peopke, killing two of them."
Reports said the powerful explosion also caused massive damage to nearby buildings.
Witnesses said after the car bomb explosion, at least two gunmen dressed in police uniform and wearing explosive vests tried to reach the headquarters main building, but were killed by guards.
No one has claimed responsibility for the attack.
Kirkuk, some 180 kilometers north of Baghdad, is an ethnically mixed city of Arabs, Kurds and Turkomans.
The area is at the center of a dispute over oil and land rights between Baghdad's central government and the autonomous Kurdish region in the north.
There have been three major attacks in recent weeks in or near Kirkuk.
On January 16, twin car bombs in Kirkuk killed more than 30 people, while days later a suicide bomber killed at least 26 people attending a funeral at a Shi'ite mosque in the nearby town of Tuz Kurmatu.
The spike in violence comes amid a political crisis between Iraq's minority Sunni Muslims and the Shi'ite-led government.
Thousands of Sunni protesters have been rallying for weeks against Shi'ite Prime Minister Nuri al-Maliki demanding a repeal of laws they claim target Sunnis.
With reporting by Reuters, AFP, dpa, and the BBC