Four Russian warships in the Caspian Sea launched 26 rockets at Islamic State in Syria which hit their targets, Russia's Defense Minister Sergei Shoigu and President Vladimir Putin said in a joint television appearance on Wednesday.

Russia started its air campaign in Syria saying it would target militant group Islamic State. But its planes have also bombed other rebel groups opposed to Syrian President Bashar al-Assad, Moscow's ally.

Western countries, Arab states and Turkey, who are waging their own bombing campaign against Islamic State but also want Assad to leave power, say Moscow is using Islamic State as a pretext to target Assad's other foes.

Russia says the Assad government should be the centerpiece of international efforts to combat extremism.

Putin said it was too early to talk about the results of Russia's operations in Syria and ordered Shoigu to continue cooperation with the United States, Turkey, Saudi Arabia, Iran and Iraq on Syria.

On Wednesday the Syrian army and allied militia carried out ground attacks on insurgent positions in Syria backed by Russian air strikes.

Putin also said that French President France Hollande had voiced the idea of uniting Assad's forces with the so-called Free Syrian Army to fight Islamic State.

But a source close to Hollande denied he had said this.

"The president spoke of the necessary presence of the Syrian opposition around the negotiating table." a source close to Hollande said. "The rest is not a French idea."

Shoigu said that on Tuesday Russia had summoned foreign military attaches in Moscow and suggested they supply Russia with any intelligence on Islamic State positions.

"Today we are expecting a reply from our colleagues and we hope they will tell us about those targets which they have," he said.

Shoigu also said Russia was ready to agree a document with the United States to coordinate actions in Syria.