Kurdish fighters backed by Iraqi security forces and U.S.-led airstrikes began a major offensive to push Islamic State militants from Sinjar, a northern Iraqi city near the Syrian border captured by the jihadist group last year.

About 7,500 peshmerga are amassing on three fronts to cordon off Sinjar and establish a buffer zone to protect the city from artillery, the Kurdistan Region Security Council said Thursday. They’re also trying to clear portions of an east-west highway used by the group to funnel equipment and fighters from Syria into Mosul, a key Iraqi city controlled by Islamic State since last summer.

As Islamic State swept through northern Iraq, it forced members of the Yezidi religious minority it considers apostates to flee into the mountains around Sinjar. Human-rights monitors accused the militants of systematic rape and carrying out other acts of sexual violence against Yezidi women and girls.

The ground assault was launched at 7 a.m. local time, following a night of airstrikes and an early morning artillery barrage. Islamic State recently moved additional fighters into Sinjar, where they now probably have about 700 members, according to the KRG. Fierce fighting is underway, the Kurdish Rudaw news agency reported.

The U.S.-led coalition struck dozens of Islamic State positions ahead of the offensive, the KRG said. The Combined Joint Task Force, under U.S. Central Command, confirmed it was supporting what it called a "major operation," in an e-mailed statement Thursday.