Russia has unleashed cruise missiles from warships in the Caspian Sea at targets across Syria, the defence minister says, as Moscow kept up its intensified bombardments in the war-torn country.

Moscow fired 18 missiles from ships in its Caspian Sea fleet at seven targets in the Raqa, Idlib and Aleppo provinces, defence minister Sergei Shoigu was quoted as saying by Russian news agencies on Friday.

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It was the second time that warships have been used since the start of the bombing campaign on September 30.

Moscow has stepped up its strikes in Syria with long-distance bombers after confirming for the first time on Tuesday that a bomb downed a Russian airliner in Egypt last month.

Shoigu told President Vladimir Putin in a briefing that cruise missile strikes against one target near the Islamic State-controlled city of Deir Ezzor had killed “more than 600 fighters”, but did not specify when the strike had taken place.

At least eight people were killed in at least 50 air strikes in the eastern Deir Ezzor province Friday, during which dozens of oil tankers were destroyed, the Syrian Observatory for Human Rights said.

Russia has also doubled the number of jets it has based in government-held territory in Syria to 69 over the past few days, Shoigu said.

Putin praised the Russian operation in Syria but said it was “still not sufficient” to wipe out the jihadists in the country and that a “large volume of work” lay ahead.

Russia is bombing in Syria at the request of its longstanding ally President Bashar al-Assad, while a US-led coalition is conducting its own air campaign against IS.