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

Hours into the operation, the Kurdish Regional Security Council said forces were in control of a section of Highway 47, of one of IS’s most active supply lines.

The advance completely isolates members of the IS group in Sinjar from militant strongholds in Syria and northern Iraq. The Kurdish fighters also said they had secured the villages of Gabarra, on the western front, and Tel Shore, Fadhelya and Qen on the eastern front.

Some 7,500 peshmerga fighters were closing in on Sinjar from three fronts, the security council said in a statement. In addition to taking the town and the highway, Operation Free Sinjar aimed to establish “a significant buffer zone to protect the city and its inhabitants from incoming artillery.”

Heavy gunfire broke out early Thursday as peshmerga fighters began their approach amid 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.

The coalition said 24 airstrikes were carried out over the past day, striking nine militant tactical units, nine staging areas and destroying 27 fighting positions, among other targets. Coalition aircraft have conducted more than 250 airstrikes over the past month across northern Iraq.

The Peshmerga “troops are holding their position, waiting for reinforcements and more airstrikes so they can then move into the center of the town. Airstrikes have been very important to the operation getting to the point where it is now,” said Maj. Gen. Seme Busal, commander of one of the front lines.

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.

The major objective of the 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.