An ally of President Vladimir Putin on Tuesday outlined the scenario of a nuclear strike on Ukraine, saying that the US-led NATO military alliance would be too scared of a "nuclear apocalypse" to directly enter the conflict in response.
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 last week ordered Russia's first mobilization since World War Two and backed a plan to annex swathes of Ukraine, warning the West he was not bluffing when he said he would 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.
Medvedev's remarks quoted the exact terminology of one of the conditions of Russia's nuclear strike doctrine: "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened".
The 57-year-old Putin ally, who once presented himself as a reformer who was ready to work with the United States to liberalize Russia, has recast himself in recent months as the most publicly hawkish member of Putin's circle.
Since Russia invaded Ukraine, he has repeatedly raised the threat of nuclear chaos and used insults to describe the West.
Washington has not detailed what it would do if Putin ordered what would be the first use of nuclear weapons in anger since the United States unleashed the first atomic bomb attacks on the Japanese cities of Hiroshima and Nagasaki in 1945.
National Security Adviser Jake Sullivan said on Sunday that the United States would respond decisively to any Russian use of nuclear weapons against Ukraine and has spelled out to Moscow the "catastrophic consequences" it would face.
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.
Russia has 5,977 nuclear warheads while the United States has 5,428 warheads, while China has 350, France has 290 and the United Kingdom has 225, according to the Federation of American Scientists.
Medvedev's comments come as Russia prepares to annex large swathes of Ukrainian territory after referendums in Russian-controlled regions in Ukraine which Ukraine and the West have described as an illegal sham.
Diplomats say Russia's nuclear saber rattling indicates Putin is trying to scare the West into reducing its support for Ukraine by hinting at using a tactical nuclear weapon to defend the annexed territories of Ukraine.
Tactical nuclear weapons, simply a nuclear device used on the battlefield, typically have much smaller explosive power than the vast strategic nuclear warheads which Russia and the United States point at each other's major cities.
"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.
When describing a possible strike on Ukraine, a Slavic neighbor which Putin describes as an artificial historical construct, Medvedev said NATO would not get involved in such a situation.
"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."