Russian jets launched a fourth day of air raids in Syria Saturday, a monitor said, hitting the Islamic State group's main stronghold after claims Moscow was instead targeting moderate rebel factions.

"Several Russian strikes hit ISIS positions west of Raqa overnight and explosions were heard in the city," Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told AFP.

Raqa has acted as the extremist group's de facto Syrian "capital" since 2013.

Russian warplanes killed 12 jihadists in Raqa province in Thursday strikes, the Observatory said, after Moscow had come under fire from the United States for allegedly targeting moderate rebel groups in Syria.

Strikes ordered by Russia -- a staunch ally of Syrian President Bashar al-Assad -- have hit several provinces since Wednesday, including Aleppo in the north, Idlib in the northwest and Hama in central Syria.

The Russian defence ministry and the Kremlin said its planes bombed ISIS targets six times Friday and also hit the group's jihadist rival, Al-Qaeda's Syrian affiliate Al-Nusra Front.

US President Barack Obama later warned that Russia's aggressive military campaign in support of Assad was a "recipe for disaster".

He told reporters that Russia "doesn't distinguish between (ISIS) and a moderate Sunni opposition that wants to see Mr Assad go."

A US-led coalition has been bombing ISIS positions in Syria since September last year.

Russia said its sorties on Friday had hit an ISIS "command post and communications centre" in Daret Ezza in Aleppo province, as well as bunkers and weapons depots in Maaret al-Numan and Habeet in Idlib province.

Russian raids also hit "an ISIS command post" in Kafr Zeita in Hama province.

According to the Observatory, ISIS does not control any of these areas.

Abdel Rahman had told AFP that seven civilians, including two children, were killed in Russian air attacks on Idlib province late Thursday.