US-backed Syrian fighters fought Islamic State (IS) militants in the heart of Raqqa, the extremists’ self-styled capital, as scores of civilians fled areas controlled by the group.

The Syrian Democratic Forces (SDF), a Kurdish-led group, has been one of the most effective forces fighting IS in Syria, but has also clashed with Turkish-backed Syrian forces elsewhere in the country.

As it battled IS in Raqqa, the SDF also fought Turkish-allied Syrian forces in Ein Daqna, in the neighbouring Aleppo province, according to Syrian activists and Turkish media.

Syrian Democratic Forces
US-backed Syrian Democratic Forces fighters talk to each other on the front line on the western side of Raqqa, north-east Syria (Hussein Malla/AP)

The SDF, aided by the US-led coalition, launched its offensive to capture Raqqa on June 6, and has since taken several areas.

The Britain-based Syrian Observatory for Human Rights said Monday’s fighting is concentrated in Raqqa’s south-western neighbourhood of Yarmouk as well as a central area close to the Old City.

The SDF says intense fighting is under way in central Raqqa, adding that its fighters have taken positions near a centuries-old mosque known as the Old Mosque.

The Kurdish-run Hawar news agency says some 180 civilians were able to flee areas controlled by IS, while the Observatory put the number in the hundreds.

The SDF said 11 IS fighters have been killed in the clashes since Sunday.

The IS-linked Aamaq news agency said 14 SDF fighters were killed in the fighting in Raqqa on Sunday alone.

The intensification of fighting comes a week after Iraqi forces declared victory against IS in the northern Iraqi city of Mosul, the largest the extremists have held.

Fighters observe a street controlled by IS
Fighters use their video camera through a hole as they film a street controlled by Islamic State militants in Raqqa (Hussein Malla/AP)

The loss of Raqqa would deal a major blow to IS, but the group still holds wide areas of the eastern province of Deir el-Zour, bordering Iraq.

That province figures to be the backdrop to the next phase of the war on IS, with government forces backed by Russia and Iran approaching from the west and south.

Pro-government forces reached the edges of the Bashari mountain range on Monday, after seizing the Zamla natural gas field one day earlier, the Observatory and Syrian military media reported.

The advance secures further natural resources revenues for the government and puts government forces in a position to penetrate the Deir el-Zour countryside.