US-backed fighters surround IS-held Syria town: Monitor

US-backed fighters have surrounded a key militant-held town in northern Syria, a monitor said on Thursday, as they press their assault towards the Islamic State group's bastion of Raqqa. 

The Syrian Democratic Forces (SDF) have sealed off the last route leading out of IS-held Tabqa, according to the Syrian Observatory for Human Rights.

"Tabqa is now surrounded, and IS will now have to use either tunnels or the Euphrates River to move around," said Observatory head Rami Abdel Rahman. 

Backed by US-led coalition air strikes and special forces advisers, the SDF is bearing down on Tabqa and the adjacent dam, which lies on the Euphrates River. 

By Thursday, the SDF had also seized the village of Safsafah, about 6km east of Tabqa, according to the observatory and an SDF statement.

"We have liberated Safsafah after 38 hours of consecutive fighting," a statement by the SDF's "Wrath of the Euphrates" campaign said.

"We are still clearing the town," it added.

The battle for Tabqa is part of the SDF's flagship campaign for Raqqa, the Syrian heart of IS's so-called caliphate, about 55km to the east. 

At their closest point, they are just eight kilometres from Raqqa city. 

IS is under pressure on several fronts, with government forces attacking it elsewhere in Syria and a US-backed offensive on its Iraqi stronghold of Mosul. 

In Syria, it still controls territory in the oil-rich eastern province of Deir Ezzor, where the observatory said IS militants had killed at least 33 people execution-style near the town of Mayadeen.

"They were all between the ages of 18 and 25," the monitor said, but it was unclear what they had been accused of. 

"The mass killing took place on the edge of a ditch that was dug by IS," the observatory added.

The SDF launched its offensive for Raqqa in November and has since captured most of the surrounding northern province. 

The Pentagon is arming and training the SDF, but questions remain over how much support the US should give the Kurdish component of the alliance given concerns from Turkey, which views the Kurdish fighters as "terrorists".