IS, formerly known as Isis, has expanded its reach into large parts of eastern Syria and northern Iraq in recent months.

The US has launched limited airstrikes against the group in Iraq but has not targeted them in Syria.

The Syrian Observatory for Human Rights, a UK-based activist group with opposition ties, said clashes around the airbase were ongoing but added that the base was under IS control.

Hundreds of soldiers and IS fighters had died in the past few days' fighting around the base, the Observatory said.

Syrian state television confirmed that government troops had lost control of the base.

"After heavy fighting by the forces defending the Tabqa airbase, our forces implemented a regrouping operation after the evacuation of the airbase," it said.

Government forces were conducting airstrikes on the base after the troops evacuated, it reported.

One year on from the chemical weapon attack in Damascus, the war in Syria has become part of a wider regional struggle.

The base lies around 45km (30 miles) outside Raqqa city, the main powerbase of IS, and is believed to hold dozens of warplanes, helicopters, tanks and artillery.

Analysts say the fall of the base is a significant blow for the Syrian government's ability to project air power into the north of the country.

BBC News