Kurdish Iraqi fighters, backed by the U.S.-led air campaign, launched an assault Thursday aiming to retake the strategic town of Sinjar, which the Islamic State overran last year in an onslaught that caused the flight of tens of thousands of Yazidis and first prompted the U.S. to launch airstrikes against the militants.

A statement from the Kurdish Regional Security Council Thursday said some 7,500 peshmerga fighters are closing in on the mountain town from three fronts in an effort to take control of the town and cut off a strategic supply line used by the Islamic State militants. The statement also says the Kurds wish to establish "a significant buffer zone to protect the city and its inhabitants from incoming artillery."

The major objective of the offensive is to cut off one of IS's most active supply lines, Highway 47, which passes by Sinjar and indirectly links the militants' two biggest strongholds — Raqqa in Syria and Mosul in northern Iraq — as a route for goods, weapons and fighters. Coalition-backed Kurdish fighters on both sides of the border are now working to retake parts of that corridor.

Warplanes in the U.S.-led coalition have been striking around Sinjar ahead of the offensive. Earlier Thursday, intense airstrikes pounded Sinjar as various Kurdish militia groups made their way over the mountain to begin the offensive.

Sinjar was captured by the Islamic State group in August 2014 shortly after the extremists seized Iraq's second-largest city, Mosul, and blitzed across northern Iraq.

In the Sinjar area, the group inflicted a wave of terror against the minority Yazidi community, members of an ancient religion whom the Islamic State group views as heretics and accuses of worshipping the devil. Tens of thousands of Yazidis fled into the mountains, where the militants surrounded them, leaving them trapped and exposed in the blazing heat. The crisis prompted the U.S. to launch air drops of aid to the stranded, and then on August 8, it launched the first round of airstrikes in what would mark the beginning of a broader coalition effort to battle the militant group in Iraq and Syria.

Various Kurdish militias on the town's edge have been fighting in guerrilla battles for months with IS fighters in Sinjar. The factions include the Turkey-based Kurdish Workers' Party (PKK), the Syria-based People's Protection Units better known as the YPG, and Yazidi-led forces billing themselves as the Sinjar Resistance. Iraqi peshmerga have also held positions further outside the town.