At least 12 members of the Islamic State group have been killed in Russia's first air strikes on the jihadist faction's main Syrian bastion, a monitoring group said Friday.

The attacks, which began Wednesday, have also targeted the Army of Conquest, the most powerful Islamist coalition battling Syrian regime forces in the northwest, a security source said.

The defence ministry in Moscow said day three of its attacks saw six air strikes against IS targets.

"Six strikes were conducted on targets of the ISIS terrorist group in 14 flights by Sukhoi Su-34, Sukhoi Su-24M and Sukhoi Su-25 jets," the ministry said, referring to the jihadist group.

Earlier it confirmed carrying out strikes on Raqa province on Thursday, as well as raids on the provinces of Aleppo in the north, Idlib in the northwest and Hama in central Syria.

The strikes are the first time Russia has targeted IS's stronghold in Raqa province, the de facto Syrian capital of its self-styled "caliphate".

The defence ministry said Su-34 planes hit "an IS training camp near the village of Maadan Jadid", 70 kilometres (45 miles) east of Raqa city, and "a camouflaged command post at Kasrat Faraj, southwest of Raqa".

The Syrian Observatory for Human Rights said those strikes had killed at least a dozen IS fighters.

"Last night, Russian strikes on the western edges of Raqa city, and near the Tabqa military airport, killed 12 IS jihadists," Observatory head Rami Abdel Rahman said.

Activists and Raqa residents said IS had cancelled Friday prayers and emptied mosques there, fearing further attacks.

"The residents are very afraid, especially if the Russians are going to operate like regime planes by targeting civilians," said activist Abu Mohammad from Raqa.

Speaking to AFP via the Internet, he said residents were staying indoors, and IS "has started to implement a plan to turn off electricity in a number of areas of the city when planes are overhead".

The Russian defence ministry said raids destroyed "a command post and communications centre" held by IS in Daret Ezza in Aleppo province and "completely destroyed" bunkers and weapons depots in Maaret al-Numan and Habeet in Idlib province.

- ' IS command post' -

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

According to the Observatory, IS does not control any of these areas. Kafr Zeita is held by Islamist rebels, while the areas in Idlib and Aleppo are held by IS's jihadist rival, Al-Qaeda Syrian affiliate Al-Nusra Front.

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

A security source said Friday that "Russian planes conducted several strikes on military positions and command centres held by the Army of Conquest in Jisr al-Shughur... and Jabal al-Zawiya in Idlib".

The Army of Conquest fiercely opposes both IS and President Bashar al-Assad's regime.

Headed by Al-Nusra, it controls all of Idlib province and has advanced on Assad's provincial stronghold of Latakia further west.

The Observatory confirmed that four Russian strikes had targeted Jabal al-Zawiya.

It said raids on Khan Sheikhun in Idlib province had killed two Al-Nusra fighters, and that Latamina in Hama province had been hit as well.

A Syrian military source told AFP Russian strikes on Friday targeted an ancient Christian town in Homs province seized by IS on August 5.

"Russian warplanes struck Al-Qaryatain this morning," the source said.

On Friday, IS launched a fierce attack on Syria's eastern city of Deir Ezzor and a nearby military airport, the Observatory said.

IS controls most of Deir Ezzor province and part of the city, but has tried for months to seize all of the provincial capital.

The latest violence came amid concerns that Russia is concentrating on groups opposed to Assad's embattled regime, instead of specifically targeting IS.

Moscow insists it is targeting IS and "other terrorist groups".

Speaking to France's Europe 1 radio, Alexei Pushkov, the head of the foreign affairs committee of Russia's lower house of parliament, refuted suggestions from Western nations that Russian planes were mainly bombing groups opposed to Assad.

"The main target are the Daesh (IS) groups situated closest to Damascus," he insisted.

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