SINJAR, Iraq — Kurdish fighters launched an assault Thursday aiming to retake the ISIS-held strategic town of Sinjar.

ISIS overran Sinjar last year in an onslaught that caused the flight of tens of thousands of Yazidis and prompted the U.S. to launch airstrikes against the militants.

Smoke rises after U.S.-led airstrikes in Sinjar, Iraq, on Thursday. Reuters

A statement from the Kurdish Regional Security Council said some 7,500 peshmerga fighters were closing in on the mountain town from three fronts. It added that Operation Free Sinjar is aimed at establishing "a significant buffer zone to protect the city and its inhabitants from incoming artillery."

Peshmerga fighters and the militants exchanged heavy gunfire early Thursday as Kurdish fighters began their approach amid heavy aerial bombardment. An Associated Press team saw a small American unit at the top of a hill along the front line calling in and confirming airstrikes.

Pentagon officials told NBC News early Thursday that 20 U.S. strikes had been launched overnight — hitting ISIS fighting positions, staging areas, a bunker and other targets. They said that U.S. special operations forces were also embedded as advisers, adding that they were "well back from the fighting.”

U.S. military officials said "hundreds" of ISIS fighters were believed to be in Sinjar.

Hours into the operation, the security council said Kurdish forces had successfully secured the village of Gabarra and controlled a section Highway 47, which is one of ISIS' most active supply lines. They also captured the villages of Gretishore and Fadhellya on the eastern front.

A spokesman for the peshmerga forces reported heavy fighting, but added: "Sinjar is completely surrounded from all directions at the moment — no support or aid will reach ISIS militants inside Sinjar."

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

ISIS inflicted a wave of terror against the minority Yazidi community, members of an ancient religion whom the militants view as heretics and accuse of worshipping the devil. An untold number were killed in last year's assault, and hundreds of men and women were kidnapped — the women enslaved and given to militants across the group's territory in Iraq and Syria, many of the men believed killed, others forced to convert.

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.

For Yazidi forces taking part, the battle is very much about retribution.

Hussein Derbo, the head of a peshmerga battalion made up of 440 Yazidis, said the men under his command could have migrated to Europe but chose to stay and fight. "It is our land and our honor. They (ISIS) stole our dignity. We want to get it back," he told Reuters.

Derbo's brother, Farman, echoed the sentiment, saying he hoped the militants did not retreat from battle so that the Yazidis could kill them all.

The major objective of the latest offensive is to completely cut off 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 and strikes grew more intense at dawn Thursday as bombs pounded targets outside the town. But Sinjar, located at the foot of Sinjar Mountain about 30 miles from the Syrian border, is not an easy target. One attempt by the Kurds to retake it stalled in December.

Sinjar is part of the disputed territories to which both the Iraqi federal government and regional Kurdish authorities lay claim.