At least 16 civilians, among them eight children, were killed on Friday in air strikes on the northern Syrian city of Raqqa by unidentified planes, a monitoring group said.

The Syrian Observatory for Human Rights said five women were also among the dead in Raqqa, the main Syrian bastion of the Islamic State jihadist group.

"The death toll could rise as there are around 40 wounded and missing people," the Observatory said.

The Britain-based Observatory typically identifies aircraft by flight patterns and munitions used.

However, it said Friday's strikes could have been carried out "by either the Syrian regime, the Russians, or the coalition led by Washington," referring to the US-led effort to strike IS in Syria and Iraq.

But Aamaq, a self-styled news agency linked to IS, said strikes by Russian warplanes had left 17 people dead in Raqqa on Friday.

On Monday, Moscow announced it would withdraw the bulk of its air force from Syria after a nearly six-month air war in support of President Bashar al-Assad.

It warned that it would continue targeting "terrorist groups".

Russia's military said Friday it was still conducting about two dozen combat sorties per day to back a government offensive to retake the ancient city of Palmyra in central Syria from IS.