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Warplanes pounded the Islamic State group's de facto Syrian capital Raqqa on Tuesday, killing at least 23 people including 13 extremists, a monitoring group said.

The British-based Syrian Observatory for Human Rights said either the regime or Russian airforce was believed to have carried out the strikes.

"Thirteen members of the Islamic State group and 10 civilians were killed in at least 16 air raids on the group's positions and premises in several parts of Raqqa city," Observatory chief Rami Abdel Rahman said.

He said the toll relied on information from only one hospital in the area because of security restrictions and that the number of dead could be higher.

IS seized Raqqa in January 2014, expelling various rebel groups who had seized it from the regime the previous year.

Since then it has become the de facto Syrian capital of the territory controlled by the group, which it dubs an Islamic "caliphate".

The town has been targeted by Syrian government airstrikes and raids launched by Russia since it began an aerial campaign in late September.

A US-led coalition fighting IS has also bombed the city in the past.

Elsewhere, the Observatory said a Kurdish-Arab alliance had advanced against IS in the northeastern province of Hassaka.

The Observatory said at least seven IS fighters had been killed in clashes with the Syrian Democratic Forces and airstrikes by the US-led coalition on Tuesday.

The SDF said Saturday it had begun its first offensive since announcing the coalition last month against IS in Hassaka province.