The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) has recaptured large areas of territory in the northern Syrian province of Raqqa that it recently lost to forces loyal to President Bashar al-Assad, opposition activists have said.

The UK-based Syrian Observatory for Human Rights said on Tuesday that ISIL forces had pushed government forces some 40km from Tabqa, an area west of Raqqa city that has a dam and an airbase.

Government troops and allied militiamen had reached within 7km of the airport on Sunday, according to the observatory.

READ MORE: Clashes rage over Syria's Tabqa air base

Tabqa airbase was the last position held by government forces in Raqqa province before ISIL overran it in August 2014, killing scores of detained soldiers in a massacre they documented on video.

The ISIL-linked Aamaq news agency posted a video showing the armed group in control of Thawra oilfield as warplanes conducted air strikes nearby. Government forces had seized the field on Sunday, only to lose it hours later.

ISIL has been under pressure in Iraq, Syria and Libya in recent weeks, but the gains in Raqqa show it is still able to take on Syrian troops backed by Russian warplanes.

According to defence analysts at think-tank IHS Jane ISIL lost about 14 percent of its territory in 2015, while Syria's Kurds almost tripled theirs.

More than five years since the conflict started, more than 270,000 Syrians have been killed in the fighting, and almost 11 million Syrians - half the country's prewar population - have been displaced from their homes.