MOSCOW - Russian airstrikes destroyed an Islamic State command post near Raqa as well as an underground bunker, the defence ministry said on Saturday, as Moscow pressed along with its bombing campaign in Syria.

"Over the past 24 hours SU-34 and SU-24M jets of the Russian airborne formation in Syria made more than 20 sorties over nine Islamic State infrastructure facilities," the defence ministry said.

A command post in the area of ISIS stronghold Raqa as well as an underground bunker storing explosives had been destroyed, it added.

A monitor said Saturday that Russian jets launched a fourth day of air raids in Syria, hitting the Islamic State groups 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, said.

Raqa has acted as the extremist groups 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 groups jihadist rival, Al-Qaedas Syrian affiliate Al-Nusra Front.

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

He told reporters that Russia "doesnt distinguish between (ISIS) and a moderate Sunni opposition that wants to see Mr Assad go."

A US-led coalition has been bombing IS 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 IS command post" in Kafr Zeita in Hama province.

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

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