Warplanes believed to have been sent by the U.S.-led coalition struck positions held by Islamic State militants near a Syrian border that beleaguered Kurdish forces have been struggling to defend.

The airstrikes on Tuesday came after fighters from the extremist group managed to enter some neighborhoods of Kobani, a strategic town near the Turkish border. Journalists on the Turkish side of the border heard the sound of warplanes before two large plumes of smoke billowed just west of Kobani.

The U.S.-led coalition has launched several airstrikes over the past two weeks near Kobani in a bid to help Kurdish forces defend the town, but the strikes appear to have done little to slow the Islamic State offensive.