The main Syrian Kurdish force fighting Islamic State militants in northern Syria claimed on Friday that it has enough fighters to take the extremists’ de facto capital of Raqqa with the help of the U.S.-led coalition – remarks that reflected a veiled warning to Ankara and also to rival, Turkey-backed opposition forces making headway toward the city. The comments by Cihan Sheikh Ehmed, the spokeswoman of the Syria Democratic Forces, came as U.S. troops are playing a bigger role on the ground in the battle to conquer Raqqa in northern Syria.