Erbil, Reuters—Kurdish forces drove Islamic State of Iraq and Syria (ISIS) militants back from the oil-rich city of Kirkuk in northern Iraq on Monday, in an advance backed by heavy airstrikes from the US-led coalition fighting the extremist group.

Speaking to a local television channel near the frontline, Kirkuk governor Najmiddin Karim, who was wearing a helmet, said the purpose of the offensive was to secure Kirkuk, which the Kurds have held since last summer.

Kurdish fighters retook around 40 square miles (100 square kilometers), including about a dozen villages, from ISIS to the south and west of Kirkuk, killing some 100 militants, a statement from the region’s security council said.

“This morning we launched an attack on three axes,” Maj. Gen. Omar Saleh Hassan told Reuters by telephone from the frontline near Tel Alwad. “Our advances are continuing.”

He said his forces faced little resistance from the militants, who are also fighting to hold the city of Tikrit around 68 miles (110 kilometers) southwest of Kirkuk as Iraqi forces close in.

Just north of Tikrit, home city of Saddam Hussein, Iraqi security forces and Shi’ite militia fighters began an offensive to regain control over the town of Al-Alam.

Military commanders said some of the attacking force were ferried across from the west bank of the Tigris river, while others were approaching from other directions.

“We have confirmed information from inside Al-Alam that a few [ISIS] fighters are still inside, mostly suiciders, and this is why we attacked them from multiple directions in order not to give them time to catch their breath,” Al-Alam mayor Laith Al-Jubouri said.

Jubouri, who has spent time with the attacking forces outside Al-Alam, said clashes were continuing in the south, west and north of the town.

In the Kirkuk offensive, the Kurdish Peshmerga destroyed four suicide car bombs and a fifth was hit by a coalition airstrike, according to a Kurdistan Security Council statement.

“In addition, Peshmerga forces have successfully controlled the road between Maktab Khaled Bridge and the Wadi Neft intersection—a key junction linking Mosul to Kirkuk, further disrupting the enemy’s freedom of movement,” the statement read.

The Kurds took full control of Kirkuk last August as the Iraqi army collapsed in the north and ISIS militants overran almost a third of the country.

But the city has remained vulnerable, with the frontline no more than 12 miles (20 kilometers) away in some places and only an irrigation canal separating the two sides. In late January, ISIS briefly overran Kurdish defenses around Kirkuk.

Monday’s gains bring the Peshmerga closer to the ISIS stronghold of Hawijah, where black-clad militants recently paraded the bodies of what they said were Shi’ite militiamen they had killed.