Suspected Islamic State militants attacked several buildings and a power station in the northern Iraqi oil city of Kirkuk in the early hours of Friday, killing eight people, and some of the attackers remained holed up in a mosque and a hotel, security sources said.

Six members of the security forces were killed along with two Iranians who were part of a team carrying out maintenance in a power station outside the city, according to a hospital source.

At least eight militants were also killed, either by blowing themselves up or in clashes with the security forces, the security sources said. Kurdish forces had dislodged the militants from all the buildings they had seized except a hotel and a mosque, where fighting continued.

Residents in the city said they had been hearing explosions since 1 am (Thursday 2300 GMT).

Islamic State claimed the attacks in online statements, and authorities declared a curfew in the city.

Crude oil production facilities were not targeted and the power supply continued uninterrupted in the city, they said.

Kurdish Peshmerga fighters took control of Kirkuk in 2014, after the Iraqi army withdrew from the region, fleeing an Islamic State advance through northern and western Iraq.

The attacks in Kirkuk came four days after Kurdish and Iraqi forces started an offensive with the backing of a US-led coalition to take back Mosul, the jihadist militia's last major city stronghold in Iraq.

The hardline group also controls part of Syria.