Syrian troops backed by Russian air strikes surge towards IS-held Tabqa

Pro-government Syrian fighters backed by Russian air power advanced on Sunday towards a key town and adjacent air base held by the Islamic State (IS) group, a monitor said.

Government forces were now within seven kilometres of the IS-held airport at Tabqa, the Syrian Observatory for Human Rights said.

"There are fierce clashes now and at least nine pro-government fighters were killed," said the Observatory's Rami Abdel Rahman. He had no immediate toll for the number of IS militants killed. 

The war in Syria - which began with the repression of anti-government demonstrations in 2011 - has killed more than 280,000 people and displaced millions. It has evolved into a brutal, complex civil war among the government, rebels, IS militants, and Kurds, each carving out zones of control.

Government fighters had been stuck at about 15 km from the airport for more than a week, but intensifying Russian air strikes on Sunday and reinforcements from the elite Republican Guard unit allowed them to advance, the Observatory said. 

Earlier this month, government fighters backed by Moscow began pushing north towards Tabqa, whose dam, military air base, and strategic transit town have been held by IS since 2014. 

Recapturing Tabqa would sever western supply route to Islamic State's de facto capital of Raqqa

The town lies some 50 kilometres west of IS's de facto Syrian capital of Raqqa city, and recapturing it would sever the supply road from the west.

The Kurdish-led Syrian Democratic Forces (SDF), backed by US air strikes, were also pushing towards Tabqa, according to Al Jazeera.

The SDF, an alliance of Kurds and Arabs, seeks to retake the northern Syrian town of Manbij from IS as part of its offensive in Raqqa province, Al Jazeera said. Manbij is strategically important because it controls a supply route from the Turkish border to Raqqa.

Clashes between SDF forces, backed by US-led air power, and IS militants raged on Sunday on the outskirts of Manbij, the UK-based Syrian Observatory for Human Rights said. No casualties were reported.

Manbij has been under IS control since 2014.