US-backed fighters have advanced to the edge of a key extremist-held town in northern Syria, a monitor said Saturday.
The Syrian Democratic Forces (SDF), an Arab-Kurdish alliance supported by US-led coalition air strikes and special forces advisers, surrounded Tabqa in early April.
The town and nearby major dam are considered a key prize in a broader offensive for Raqqa, the de facto Syrian capital of ISIS's self-proclaimed "caliphate", about 55 kilometers (34 miles) to the east.
SDF fighters "are now hundreds of metres (yards) from Tabqa", said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a British-based monitoring group.
The alliance was reported to have advanced overnight after driving the extremists from two areas just southeast and southwest of the town.
"Heavy fighting is taking place in the vicinity of the two suburbs," Abdel Rahman said. "IS is trying to counter-attack."
ISIS is under pressure on several fronts, with government forces attacking it elsewhere in Syria and a US-backed offensive targeting its Iraqi stronghold of Mosul.
The SDF launched its campaign for Raqqa in November and has since captured most of the surrounding province.