Raqqah also was the site of a failed rescue attempt earlier this summer in which Delta Force commandos sought to snatch Foley and a group of other Americans held by the Islamic State from a prison east of the city of Raqqah, according to U.S. officials and witnesses in the area.

The capture of Tabqa airfield on Sunday was the latest in a string of recent military successes for the extremist fighters, who have been consolidating their hold across northern Syria even as U.S. airstrikes launched earlier this month temper the group’s expansion in northern Iraq.

U.S. officials have not ruled out pressing the airstrikes into Syria, telling reporters last week that the United States would not be restricted by “geographic boundaries” when confronting the militants.