Armed militants have raided government buildings in the Iraqi city of Kirkuk.
Iraqi media said suicide bombers had attacked police stations and a power station, but that security forces repelled fighters.
The so-called Islamic State group said it was behind the attack, claiming its fighters had broken into the town hall and taken control of a hotel.
It comes as Iraqi government-led forces attempt to retake the IS stronghold of Mosul.
IS fighters were reported to have set fire to a chemical plant south of Mosul as they were retreating on Thursday. Sources say they started the fire at the sulphur plant in al-Mishraq deliberately when they were being pushed out of the area by the Iraqi security services during the ongoing offensive.A curfew has been imposed in Kirkuk "until further notice", Iraqi media said.
A local TV channel showed footage of black smoke rising over the city, with automatic gunfire audible.
The Beirut-based newspaper al-Sumaria reported that after the dawn attack, police had killed one suicide bomber but three others blew themselves up.
A district police chief, Brig Gen Sarhad Qadir, told the paper that the situation was under control.
It is not yet clear whether anyone other than the bombers has been killed or injured.
Kirkuk, 180 miles (290km) north of Baghdad and 105 miles (170km) south east of Mosul, is claimed both by the Iraqi government and by the country's Kurds. It is oil-rich but a report from Reuters said the attack had not affected crude oil production facilities.