Kurdish forces claimed to have pushed back Islamic State militants from a 300-square-mile area of northern Iraq on Wednesday and said they cut one of the extremist group’s key supply lines to the occupied city of Mosul.

The multi-pronged operation, which began in the early morning, involved about 5,000 Kurdish soldiers, known as peshmerga and was backed by U.S.-led airstrikes, according to a statement from the government of Iraq’s semi-autonomous Kurdish region.

The offensive came amid speculation that Iraqi forces are preparing for an assault on Mosul, one of Iraq’s biggest cities, which was seized by Islamic State extremists in June as they swept across the north. […]