Kurdish YPG forces have seized a hill inside the Syrian province of Raqqa, the Islamic State (Isis) Syrian stronghold, after deadly clashes, according to a monitoring group.

The Syrian Observatory for Human Rights reported that Kurdish troops, supported by Syrian rebel groups, have taken Baghdak Hill south of the border town Kobani which is situated within Raqqa province. IS has been driven from Kobani by the YPG and the all-female YPJ Kurdish forces, backed by US-led air strikes.

The jihadist group established its de facto capital in the city of Raqqa last year after seizing vast swaths of land straddling between Iraq and Syria. In recent months the city has been pounded by warplanes from the US-led coalition.

"It is the first time they have got into Raqqa," the Observatory spokesperson told Reuters in reference to the Kurdish forces. He said that at least 35 IS fighters and four YPG members had been killed on Sunday (15 February) in fighting near Kobani.

The Observatory said that Kurds had taken over at least 163 villages around the border town, which became key in the international efforts to defeat IS.

The opposition monitoring group also said that several hundred rebel fighters joined the Kurds in their effort against the Sunni militants.

The YPG press centre claimed that 18 IS militants were captured alive.