Russian warships joined in strikes in Syria with a volley of cruise missile attacks Wednesday as Russian President Vladimir Putin pledged his air force would back a ground offensive by government forces.

Ships from the Caspian Sea fleet launched 26 cruise missile strikes that hit 11 targets over 1,500 kilometres away in Syria, Moscow said.

Putin said Russian efforts "will be synchronised with the actions of the Syrian army on the ground and the actions of our air force will effectively support the offensive operation of the Syrian army", at a televised meeting with Defence Minister Sergei Shoigu.

The Russian leader, however, also stressed the need for cooperation with a US-led coalition fighting Islamic State jihadists, saying that without cooperation from the US, Turkey and Saudi Arabia, the intervention was unlikely to work.

Russian forces have struck 112 targets in war-torn Syria since last week launching a bombing campaign that Moscow says is targeting the IS group, Shoigu told Putin in the televised briefing.

"Strikes have hit 112 targets from September 30 until today," Shoigu said. "The intensity of the strikes is increasing."

In a sign that Russia was ramping up its involvement, Shoigu said that four Russian warships had hit sites in Syria on Wednesday with cruise missiles.

"In addition to the air force, four warships of the Caspian flotilla have been involved," Shoigu said, adding that the warships had carried out 26 cruise missile strikes against 11 targets.

A military spokesman told Russian news wires that the strikes from the warships had hit positions of IS and Al-Qaeda affiliate Al-Nusra Front.

- Rockets in the darkness -

A video released by the Russian defence ministry showed rockets being launched from a ship in darkness and traced their route to Syria over Iran and Iraq.

Russia began air strikes in Syria a week ago following a request by long-standing ally President Bashar al-Assad.

Moscow insists it is hitting IS and other "terrorist" targets, but the US and its allies fear that Moscow is aiming to bolster Assad's regime.

Putin also said that French leader Francois Hollande had suggested a possible plan to get Assad's forces to combine efforts with the Western-backed Free Syrian Army, the main moderate opposition group fighting the Damascus regime.

A Hollande aide later denied he had said any such thing. "The president spoke of the necessary presence of the Syrian opposition around a future negotiating table. The rest is not a French idea," he told reporters in Strasbourg.

"During my last visit to Paris, French President Hollande expressed an interesting idea according to which, in his opinion, it might be possible to at least try to unite the efforts of the government troops of president Assad's army and the so-called Free Syrian Army," Putin said.

A member of Hollande's entourage denied that he had suggested an alliance between the groups.

Putin met with Hollande and German Chancellor Angela Merkel to discuss a peace plan for Ukraine in Paris last Friday.