MOSCOW -- An ally of President Vladimir Putin warned NATO on Thursday that a defeat of Russia in Ukraine could trigger a nuclear war, while the head of the Russian Orthodox Church said the world would end if the West tried to destroy Russia.

Such apocalyptic rhetoric is intended to deter the U.S.-led NATO military alliance from getting even more involved in the war, on the eve of a meeting of Ukraine's allies to discuss sending Kyiv more weapons.

But the explicit recognition that Russia might lose on the battlefield marked a rare moment of public doubt from a prominent member of Putin's inner circle.

"The defeat of a nuclear power in a conventional war may trigger a nuclear war," former Russian President Dmitry 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.

Striking a similar tone at what he described as an anxious time for the country, the head of the Russian Orthodox Church said in a sermon for Epiphany that trying to destroy Russia would mean the end of the world.

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.

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.

'ALARMING TIME'

The Kremlin chief has sought in recent months to gird Russians for a much tougher battle while promising eventual victory in a war that the leaders of the West say they will never let him win.

The United States has denied Russian claims that it wants to destroy Russia, while President Joe Biden has cautioned that a conflict between Russia and NATO could trigger World War Three.

But top Putin allies say the tens of billions of dollars' worth of U.S. and European military assistance to Ukraine shows that Russia is now in a confrontation with NATO itself - the Cold War nightmare of both Soviet and Western leaders.

Patriarch Kirill, the head of the Russian Orthodox Church, said in a sermon: "We pray to the Lord that he bring the madmen to reason and help them understand that any desire to destroy Russia will mean the end of the world."

"Today is an alarming time," state news agency RIA quoted him as saying. "But we believe that the Lord will not leave Russian land."

Putin's foreign minister, Sergei Lavrov, told reporters in Minsk that Russia would do everything to ensure NATO and European Union leaders "sobered up" as soon as possible.

"I hope that the sobering up will come," Lavrov said. "We will do everything so that our colleagues from NATO and the European Union sober up as soon as possible."

NUCLEAR DOCTRINE

Russia鈥檚 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 United States 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 Russia invaded Ukraine, Medvedev has repeatedly raised the threat of a nuclear war, but his admission now of the possibility of Russia's defeat indicates the level of Moscow's concern over increased Western weapons deliveries to Ukraine.

Russia and the United States, by far the largest nuclear powers, hold around 90% of the world's nuclear warheads.

Asked if Medvedev's remarks signified that Russia was escalating the crisis to a new level, Kremlin spokesman Dmitry Peskov said: "No, it absolutely does not mean that."

He said Medvedev's remarks were in full accordance with Russia's nuclear doctrine which allows for a nuclear strike after "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened."

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Guy Faulconbridge in Moscow and Felix Light in Tbilisi