Russia's nuclear arsenal and the rules Moscow has laid out for its use are the only factors preventing the West from starting a war against Russia, a top ally of President Vladimir Putin said in an article published on Sunday.
Former President Dmitry Medvedev, now deputy chairman of Russia's Security Council, also said Moscow would pursue its war in Ukraine until the "disgusting, almost fascist regime" in Kyiv was removed and the country had been totally demilitarised.
In an interview aired separately on Sunday, Putin said Russia is ready to negotiate with all parties involved in the war, but said that Kyiv and its Western backers have refused to engage in talks.
Medvedev, who once cast himself as a liberal moderniser as president from 2008 to 2012, is one of the most hawkish proponents of the war. He regularly denounces the West, which he accuses of wanting to break Russia apart to benefit Ukraine.
"Is the West ready to unleash a fully-fledged war against us, including a nuclear war, at the hands of Kyiv?" he wrote in a 4,500-word article for the Rossiiskaya Gazeta newspaper.
"The only thing that stops our enemies today is the understanding that Russia will be guided by the fundamentals of state policy ... on nuclear deterrence. And in the event that a real threat arises, it will act on them."
Putin and other senior officials have repeatedly said Russia's policy on nuclear weapons dictates they can be used if there is a threat to territorial integrity.
Russia has the world's largest stockpile of nuclear weapons, with close to 6,000 warheads, according to experts.
Earlier this month, Putin said the risk of a nuclear war was rising, but insisted Russia had not "gone mad" and that it saw its own nuclear arsenal as a purely defensive deterrent.
"The Western world is balancing between a burning desire to humiliate, offend, dismember and destroy Russia as much as possible, on the one hand, and the desire to avoid a nuclear apocalypse, on the other," said Medvedev.
If Russia did not get the security guarantees it is demanding, he said, "The world will continue to teeter on the brink of World War Three and nuclear catastrophe. We will do everything we can to prevent it".
Medvedev also said Russia could forget about normal ties with the West for years and perhaps decades to come and would focus instead on relations with the rest of the world.