Syrian activists say airstrikes on the Islamic State's de facto capital, Raqqa, have killed at least 18 civilians. The victims include two children, aged 10 and 15 years.

The activists are blaming Russian and Syrian air force for Wednesday's airstrikes. The U.S.-led coalition has also been bombing the IS bastion in Syria.

Activists from the group known as Raqqa Is Being Slaughtered Silently, which relies on residents to smuggle news out of IS-held territory, say the air raids also wounded 28 people.

The group says at least one airstrike targeted a neighborhood popular among "foreign fighters" — militants who have migrated to Syria to fight with the extremist IS group.

Another group, the Britain-based Syrian Observatory for Human Rights, says 25 civilians died in the airstrikes, among them six children.