Former Russian President Dmitry Medvedev, an ally of Kremlin chief Vladimir Putin, warned the North Atlantic Treaty Organization (Nato) on Thursday that the defeat of Russia in Ukraine could trigger a nuclear war.
"The defeat of a nuclear power in a conventional war may trigger a nuclear war," Medvedev, who serves as deputy chairman of Putin's powerful security council, said in a post on Telegram.
"Nuclear powers have never lost major conflicts on which their fate depends," said Medvedev, who served as president from 2008 to 2012.
Medvedev said Nato and other defence leaders, due to meet at Ramstein Air Base in Germany on Friday to talk about strategy and support for the West's attempt to defeat Russia in Ukraine, should think about the risks of their policy.
Russia and the US, by far the largest nuclear powers, hold around 90% of the world's nuclear warheads. Putin is the ultimate decision maker on the use of nuclear weapons.
While Nato has conventional military superiority over Russia, when it comes to nuclear weapons, Russia has nuclear superiority over the alliance in Europe.
Putin casts Russia's "special military operation" in Ukraine as an existential battle with an aggressive and arrogant West, and has said that Russia will use all available means to protect itself and its people against any aggressor.
Russia’s Feb 24 invasion of Ukraine has triggered one of the deadliest European conflicts since World War Two and the biggest confrontation between Moscow and the West since the 1962 Cuban Missile Crisis.
The US and its allies have condemned Russia's invasion of Ukraine as an imperial land grab, while Ukraine has vowed to fight until the last Russian soldier is ejected from its territory.
Since a grim New Year's Eve message describing the West as Russia's true enemy in the war on Ukraine, Putin has sent several signals that Russia will not back down. He has despatched hypersonic missiles to the Atlantic and appointed his top general to run the war.
Putin said on Wednesday that Russia's powerful military-industrial complex was ramping up production and was one of the main reasons why his country would prevail in Ukraine.
Russia's nuclear doctrine allows for a nuclear strike after "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened".
Washington has not detailed what it would do if Putin ordered what would be the first use of nuclear weapons in war since the US unleashed the first atomic bomb attacks on the Japanese cities of Hiroshima and Nagasaki in 1945.
Medvedev, 57, who once presented himself as a reformer who was ready to work with the US to liberalise Russia, has recast himself since the war as the most publicly hawkish member of Putin's circle.
Since Russia invaded Ukraine, Medvedev has repeatedly raised the threat of nuclear chaos and used insults to describe the West.
Russia has 5,977 nuclear warheads while the US has 5,428, China 350, France 290 and the United Kingdom 225, according to the Federation of American Scientists.