Syrian opposition activists say the death toll from purported Russian airstrikes on the IS-held city of Raqqa in the past 24 hours has climbed to at least 55, mostly civilians.

The extremist group has controlled the city in northern Syrian since 2013 and considers it the capital of its self-styled caliphate.

The Britain-based Syrian Observatory for Human Rights, which documents the conflict through activists on the ground, said 55 people have been killed, including 13 children and a pregnant woman, in a series of Russian airstrikes that hit residential areas.

The anti-Islamic State activist group Raqqa is Being Slaughtered Silently also said the airstrikes were by Russian warplanes and put the number of deaths at 60.

The IS-affiliated news agency Aamaq said 43 were killed and 60 wounded.