Islamic State militants armed with assault rifles and explosives attacked targets in and around the northern Iraqi city of Kirkuk early Friday, in an assault that appeared aimed at diverting Iraqi security forces from a massive offensive against the IS-held city of Mosul.

At least 13 workers, including four Iranians, were killed when IS militants stormed a power plant north of Kirkuk and then blew themselves up.

Multiple explosions meanwhile rocked the city and gun battles were ongoing, said witnesses in Kirkuk, speaking on condition of anonymity as they were concerned for their safety. Much of the fighting was centred on a government compound in the city. They said the streets were largely deserted out of fear of militant snipers.

In this image made from video, smoke rises from a building where two militants are believed to be holed up, according to Rudaw TV, in Kirkuk, Iraq on Friday, Oct. 21, 2016. (Rudaw TV)

IS said its fighters targeted the provincial headquarters. The claim was carried by the IS-run Aamaq news agency and could not immediately be verified.

Local Kurdish television channel Rudaw aired footage showing black smoke rising over the city as extended bursts of automatic gunfire rang out. It quoted Kirkuk Gov. Najmadin Karim as saying that the militants had not seized any government buildings.

In the power plant attack, which took place in Dibis, a town north of Kirkuk, three IS suicide bombers entered the facility and took 13 workers hostage, said Maj. Ahmed Kader Ali, the Dibis police chief.

The attackers asked to be taken to the Iranians who worked at the plant, and one of the workers took them to the Iranians before escaping. The militants then killed the Iranians and the other workers, and detonated their explosive vests when police arrived, Ali said.

Iran's Foreign Ministry spokesman Bahram Ghasemi condemned the Kirkuk assault, which he said killed four Iranians and wounded three others, according to the official IRNA news agency. It was not immediately clear if Iranians were targeted in other attacks.

Kirkuk is some 170 kilometres (100 miles) from the Islamic State-held city of Mosul, where Iraqi forces have been waging a wide-scale offensive since Monday.

The oil-rich city is some 290 kilometres (180 miles) north of Baghdad and southeast of Mosul. It is claimed by both Iraq's central government and the country's Kurdish region. Kurdish forces assumed full control of Kirkuk in the summer of 2014, as Iraq's army and police crumbled in the face of a lightning advance by IS.

Later Friday, Rudaw TV said all IS militants who took part in the Kirkuk attack had been killed except for two who were holed up in a newly built hotel, which was damaged in the attack and from where they were battling Kurdish forces.

Kirkuk police commander Brig. Gen. Khattab Omer said clashes were still underway, without providing further details. There was no immediate word on casualties among civilians or Kurdish forces in Kirkuk, and the TV report could not immediately be confirmed.

Kemal Kerkuki, a senior commander of Kurdish peshmerga forces west of Kirkuk, said the town where his base is located outside the city also came under attack early Friday, but that the base was now under control.

He said IS maintains sleeper cells in Kirkuk and the surrounding villages. "We arrested one recently and he confessed," he said, adding that the attackers may have posed as displaced civilians in order to infiltrate the city.

Kirkuk province has absorbed hundreds of thousands of displaced people from neighbouring provinces since IS first overran wide stretches of northern and western Iraq in the summer of 2014, capturing Mosul, the country's second largest city.

Iraqi and Kurdish forces backed by a U.S.-led coalition launched a multi-pronged assault this week to retake Mosul and surrounding areas from IS. The operation is the largest undertaken by the Iraqi military since the 2003 U.S.-led invasion.

Iraqi officials said they had advanced as far as the town of Bartella, 15 kilometres (nine miles) from Mosul's outskirts, by Thursday.

Lt. Gen. Talib Shaghati of Iraq's elite forces held a press conference Friday a kilometre (half a mile) away from the town and insisted the special forces had "full control." He said special forces were clearing explosives and contending with some snipers who remained in the town. Gunfire could be heard in the distance.

IS, which still controls a swath of territory stretching across Syria and Iraq, has a history of launching diversionary attacks on distant fronts when it comes under pressure. In April 2015, Iraqi forces announced the liberation of the central city of Tikrit. The following month, IS militants captured the western Iraqi city of Ramadi and the eastern Syrian city of Palmyra. Both cities have since been retaken by Iraqi and Syrian forces.

Elsewhere in Iraq, the country's top Shiite cleric called on forces taking part in the Mosul offensive to protect civilians, and for residents of Mosul, a mainly Sunni city, to co-operate with security forces.

"We stress today upon our beloved fighters, as we have before on many occasions, that they exercise the greatest degree of restraint in dealing with civilians stuck in the areas where there is fighting," the reclusive Grand Ayatollah Ali al-Sistani said in a Friday sermon read by an aide. "Protect them and prevent any harm to them by all possible means."