Dmitry Medvedev, a former president who now serves as deputy chairman of Russia's Security Council, said Russia had the right to defend itself with nuclear weapons if it is pushed beyond its limits and that this is "certainly not a bluff".

Putin on Wednesday ordered Russia's first mobilisation since World War Two and backed a plan to annex swathes of Ukraine, warning the West he was not bluffing when he said he'd be ready to use nuclear weapons to defend Russia.

"Let's imagine that Russia is forced to use the most fearsome weapon against the Ukrainian regime which had committed a large-scale act of aggression that is dangerous for the very existence of our state," Medvedev said in a post on Telegram.

According to Russia's nuclear doctrine, the president may use nuclear weapons if the state faces an existential threat, including from conventional weapons.

"I believe that NATO would not directly interfere in the conflict even in this scenario," Medvedev said. "The demagogues across the ocean and in Europe are not going to die in a nuclear apocalypse."

Around 90% of the world's nuclear warheads are held by Russia and the United States, who remain by far the world's biggest nuclear powers.

"I have to remind you again - for those deaf ears who hear only themselves. Russia has the right to use nuclear weapons if necessary," Medvedev said, adding that it would do so "in predetermined cases" and in strict compliance with state policy.

Medvedev's comments come as Russia prepares to annex large swathes of Ukrainian territory after referendums in Russian-controlled regions in Ukraine. Ukraine and the West have denounced the votes as an illegal sham.

Medvedev, who cast himself as a liberalising president from 2008-2012, has regularly issued hawkish statements about the war in Ukraine.

(Writing by Caleb Davis; Editing by Alex Richardson/Guy Faulconbridge)