BEIRUT—The extremist group Islamic State captured a major air base in Syria in the northeastern province of Raqqa, driving out regime forces and gaining full control of an entire province for the first time in the country's civil war.

The takeover of the Tabqa air base on Sunday follows setbacks in neighboring Iraq, where U.S. airstrikes since Aug. 8 succeeded in beating the group back in several areas in the north.

The base had been the last foothold for forces of Syrian President Bashar al-Assad in Raqqa. Islamic State fighters over the past month drove regime forces from the headquarters of the Syrian Army's 17th Division and another base for the 93rd Brigade of the same division.

The loss of the air base triggered a wave of anger among many Assad supporters, particularly members of his own minority Alawite sect, who blamed military and security leaders for abandoning the fight and leaving many soldiers to a brutal fate at the hands of the Sunni militants.

Islamic State fighters were seen parading in the town of Tabqa with the severed heads of soldiers killed at the air base, according to the U.K.-based Syrian Observatory for Human Rights, an opposition group tracking the conflict through its own network of activists inside Syria.