Syrian opposition fighters have overrun Taftanaz airbase, the largest in northern Syria, after several days of fierce combat, the Syrian Observatory for Human Rights has said.

Anti-government activists said fighters from Jabhat al-Nusra (al-Nusra Front) and other groups opposed to President Bashar al-Assad seized control of buildings, ammunition and military equipment in the sprawling Taftanaz airbase in northern Idlib province on Friday.

"The fighting at Taftanaz military airport ended at 11:00 am (09:00 GMT) and the base is entirely in rebel hands," said Rami Abdel Rahman, the director of the UK-based rights group, on Friday.

"Many regime forces have been killed and most of the soldiers and officers fled at dawn," he told the AFP news agency by phone.

The capture of the base marks a significant advance for the rebels, who control vast swathes of Syria's north and east and are battling Syrian government forces in most major cities, including on the outskirts of Damascus.

The United States has branded Nusra a terrorist organisation although it enjoys wide support in Syria for its combat skills.

Many soldiers and officers fled the base at dawn, while the total number of casualties for each side was not immediately available.

The rebels seized several military vehicles and a major weapons depot.

The government forces, however, managed to pull out most of the 60 helicopters deployed at the airbase, leaving behind 20 choppers that are no longer in working condition, the Observatory said.

The aircraft were likely moved to the nearby Mastuma camp and the neighbouring town of Fuaa, according to the watchdog.