Kurdish forces said they drove Islamic State militants out of villages near the oil city of Kirkuk in northern Iraq on Wednesday, in an offensive backed by airstrikes from the U.S.-led coalition.
The assault consolidated their control over Kirkuk and brought the peshmerga, the military forces of Iraq's autonomous Kurdistan region, closer to the insurgents' bastion of Hawijah, around 55 km (34 miles) west of Kirkuk.
Around 3,500 peshmerga took part in the operation beginning early on Wednesday southwest of Kirkuk, the Kurdistan region's security council said in a statement.
By late afternoon, Kurdish forces had secured 140 square km (55 square miles) including a number of villages and the Ghara heights, "further diminishing ISIL's ability to attack peshmerga forces and putting additional pressure on ISIL in Hawijah."
Islamic State, which has taken control of large parts of Iraq and neighboring Syria, is also known as ISIS and ISIL.
Kurdistan's security council said at least 40 Islamic State militants had been killed in combat and others were seen fleeing the battlefield toward Hawijah and surrounding areas. Fifteen peshmerga also died, according to a source in a Kirkuk hospital.
Coalition warplanes struck 30 militant positions during the ground offensive, including three Humvees, three pick-up trucks and suicide car bomb, the security council said.
There was no immediate independent confirmation of the details of the report or comment from the coalition forces.
The Kurds took full control of Kirkuk last summer when Iraqi soldiers abandoned their bases in and around the city as Islamic State militants overran around a third of the country.
The peshmerga, who gained battlefield experience fighting Saddam Hussein's forces when the autocrat was in power, are seen by the United States and its coalition partners as a vital deterrent against Islamic State, which wants to redraw the map of the Middle East.
Iraq's army is regarded as ineffective and corrupt, and depends heavily on Iranian-backed Shi'ite militias in its efforts to contain Islamic State.
Kurdish leaders say they will never give up the ethnically mixed city, which sits outside the formal boundary of their region on some of Iraq's largest oil reserves, but to which they, as well as Turkmen and Arabs, lay claim.
The peshmerga have been widening a buffer around Kirkuk city in a series of offensives in recent months.
Apart from the gains around Kirkuk, the frontline between peshmerga and Islamic State has hardly moved for months.
Islamic State has not been able to take ground from the peshmerga since the U.S.-led coalition started bombing the insurgents.
The Kurds already control most of the territory they claim as their own, and have little incentive to push further into predominantly Arab towns and villages, except where they pose a direct threat to their region.