Russia staged air strikes on the Islamic State stronghold of Raqqa in northern Syria on Tuesday, a senior French government source said.

"At this moment, the Russians are in the process of strongly hitting the city of Raqqa, which is proof that they too are becoming conscious (of the threat from ISIS)," the source said.

The action followed President Vladimir Putin's announcement that Moscow would intensify air strikes in Syria after Russia's Federal Security Service confirmed that a bomb caused the crash of a Russian tourist airliner over Egypt's Sinai peninsula in October.

ISIS claimed responsibility for that attack as well as last Friday's attacks on restaurants, a concert hall and a sports stadium in Paris that killed 129 people.