At least 24 Islamic State group fighters and eight civilians were killed in air strikes Sunday on Syria's Raqqa, the de facto capital of IS, a monitor said.

It was unclear whether the raids were carried out by Syrian, Russian or US-led coalition warplanes, the Syrian Observatory for Human Rights said.

"At least 24 IS members, among them three commanders, as well as eight civilians were killed in air strikes," Observatory director Rami Abdel Rahman said.

"We do not know whether the warplanes were from the US-led coalition, the Syrian regime or Russia," he added.