If Ukraine is admitted into the US-led NATO military alliance, then the conflict in Ukraine would be guaranteed to escalate into World War Three, a Russian Security Council official was quoted as saying on Thursday.

Just hours after Russian President Vladimir Putin formally proclaimed the annexation of up to 18 percent of Ukraine on Sept. 30, President Volodymyr Zelensky announced a surprise bid for fast-track membership of NATO.

Full NATO membership for Ukraine is far off because all the alliance's 30 members would have to give their consent.

"Kyiv is well aware that such a step would mean a guaranteed escalation to World War Three," TASS quoted Alexander Venediktov, the deputy secretary of Russia's Security Council, as saying.

Venediktov, who is deputy to Security Council Secretary Nikolai Patrushev, a powerful Putin ally, said he felt Ukraine's application was propaganda as the West understood the consequences of Ukrainian membership of NATO.

"The suicidal nature of such a step is understood by NATO members themselves," he said.

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President Vladimir Putin has repeatedly railed against the United States for driving NATO's eastward expansion, especially its courting of ex-Soviet republics such as Ukraine and Georgia which Russia regards as part of its own sphere of influence.

Putin on Sept. 21 warned the West he was not bluffing when he said he would be ready to use nuclear weapons to defend Russia against what he said was "nuclear blackmail" from major Western powers.

US President Joe Biden has said the world faces the biggest risk of nuclear Armageddon since the 1962 Cuban Missile Crisis. NATO is due to hold an annual nuclear preparedness exercise dubbed "Steadfast Noon" next week.

Russia and the United States are by far the biggest nuclear powers: they control about 90 percent of the world's nuclear warheads.

Venediktov said Zelenskiy's call for preventive strikes against Russia was dangerous, cautioning that nuclear war would have catastrophic consequences for the world.

"We must remember: a nuclear conflict will affect absolutely the whole world — not only Russia and the collective West, but every country on this planet," Venediktov said. "The consequences would be disastrous for all mankind."