SINJAR, Iraq — Supported by U.S.-led air strikes, Kurdish Iraqi troops on Thursday seized part of a highway that is used as a vital supply line by the Islamic State group, a key initial step in a major offensive to retake the strategic town of Sinjar from the militants. Hours into Thursday’s operation, the Kurdish Regional Security Council said its forces controlled a section of 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. “By controlling Highway 47, which is used by Daesh to transport weapons, fighters, illicit oil and other commodities that fund their operations, the coalition intends to increase pressure ...