KIRKUK, Iraq — 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 centered on a government compound in the city. They said the streets were largely deserted out of fear of militant snipers.

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.

The fighting raged late into the afternoon, and heavy smoke billowed up from the area of the provincial headquarters, where clashes were underway, according to live footage on the local Terkmeneli TV channel. It was unclear what caused the blaze.

In the power plant attack, which took place in Dibis, a town north of Kirkuk, three IS suicide bombers stormed the facility and killed 13 workers, including four Iranians, before blowing themselves up as police arrived, said Maj. Ahmed Kader Ali, the Dibis police chief.