At least 16 people have been killed as suicide bombers and gunmen attacked a police HQ in the northern Iraqi city of Kirkuk, medical sources say.
Officials said militants had tried to seize the compound after a bomb exploded at the gates, but were unsuccessful. A police chief was hurt.
No group has said it carried out the attack.
Kirkuk is ethnically mixed, and at the centre of a dispute between Baghdad and the Kurds over oil and land rights.
Sunni insurgents linked to al-Qaeda have been blamed for much of the recent violence in Iraq.
However, there has been a relative calm in recent days.
'Terrifying' explosion
The attack took place at around 09:15 local time (06:15 GMT) in the centre of Kirkuk.
"I saw a vehicle stop at the checkpoint at the main entrance, and the police started checking it," eyewitness Kosrat Hassan Karim told the AFP news agency.
"Suddenly, a loud explosion happened, it was terrifying."
Two militants reportedly dressed in police uniforms and armed with guns, grenades and suicide vests rushed the main gate of the headquarters after the bomb was detonated.
One was killed immediately by police. They tried to dismantle the other's suicide vest but were unsuccessful, so they shot him dead, reports said.
At least 90 people were injured including local police chief Brigadier Sarhad Qader and the press officer of the police directorate, whose conditions are described as critical. Both have been moved to a hospital in Irbil.
Firefighting chief Brig-Gen Naseh Mohammed put the death toll as high as 30.
The attack caused massive damage to nearby buildings.
Traffic in the city centre was stopped, and offices in the area were evacuated.
With its massive oil reserves, Kirkuk is the most bitterly contested of Iraq's disputed territories.
It houses a mixture of Arabs, Kurds and Turkmen.
The Kurds want to incorporate it into their largely autonomous region, while Arabs and Turkmen oppose any change to its current status, ruled directly from Baghdad.
Correspondents say militants often exploit differences between the Iraqi and Kurdish security forces by launching deadly attacks in the city.
Two weeks ago at least 10 people were killed in a suicide bomb attack on the offices of the Kurdish Democratic Party in the city.
Days later, a suicide bomber killed at least 23 people at a Shia mosque in the nearby village of Tuz Khurmato, mainly populated by ethnic Turkmen.
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