Russian President Vladimir Putin on Friday said that any direct contact or direct clash of NATO troops with the Russian army would lead to a "global catastrophe".

Addressing the news conference in Kazakhstan's capital Astana, Putin said, "In any case, direct contact, direct clash of (NATO) troops with the Russian army is a very dangerous move which could lead to a global catastrophe. I hope that those who are saying this are smart enough not to take such a step."

Earlier, Putin had warned that he would use nuclear weapons to defend Russian territory after annexing four regions of Ukraine last month, a move which the UN this week condemned.

On Tuesday, Group of Seven (G7) nations warned that the use of nuclear weapons on Ukraine will have severe consequences, according to a statement released by the White House.

Leaders of G7 states (the UK, Germany, Italy, Canada, the US, France, and Japan) said, "We deplore deliberate Russian escalator steps, including the partial mobilization of reservists and irresponsible nuclear rhetoric, which is putting global peace and security at risk. We reaffirm that any use of chemical, biological or nuclear weapons by Russia would be met with severe consequences."

Earlier, US President Joe Biden spoke of nuclear "Armageddon" in a warning about the escalation in Russia's war in Ukraine during a speech on Thursday.

Biden warned of the "assault" on American institutions and talked of the threat of Russian President Vladimir Putin using nuclear weapons, at the New York home of James Murdoch, the son of media baron Rupert Murdoch.

"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis," Biden said in New York City.

At a fundraiser for the Democratic Senatorial Campaign Committee, Biden said Putin is "not joking when he talks about the use of tactical nuclear weapons or biological or chemical weapons." This is because, Biden said, "his military is -- you might say -- significantly underperforming,"

"I'm trying to figure out, what is Putin's off-ramp?" Biden said. "Where does he find a way out? Where does he find himself in a position that he does not only lose face but lose significant power within Russia?"

According to a pool report, Biden told attendees at the event, "I don't think there is any such thing as the ability to easily use a tactical nuclear weapon and not end up with Armageddon."

Putin has renewed his nuclear threats as he announced the annexation of Ukrainian territory, some of which Russia doesn't control, and with the call-up of 300,000 reservists to reinforce his flagging invasion.