Vladimir Putin is "unhinged" and the West should be wary of a leader who has his finger on the nuclear button, a former US intelligence chief has warned.
Ukraine is locked in a fifth day of fighting after Moscow launched a full-scale invasion - though the Russian military has so far failed to make the inroads it believed it would.
Delegates from both Ukraine and Russia are meeting on Monday on the Belarus border for peace talks.
Former US director of national intelligence James Clapper said the move could be a "positive sign", but was cautious in trusting what Russia says.
He told CNN: "This could be a sincere effort on the part of the Russians to negotiate or it could be going through the form of things and encouraging the Ukrainians to lay down their arms and give up.
"We just have to see how this pans out. I personally am not one to trust what the Russian says so we will have to see, but I do think it's a positive sign."
Watch: Russia's Putin puts nuclear forces on high alert
On Sunday, Putin announced he was putting Russia’s strategic nuclear deterrent on high alert in response to what he said was "unfriendly" steps from the West.
The Russian leader said he was putting Moscow’s nuclear forces on a “special regime of combat duty” in response to “aggressive statements” coming from Western powers.
Clapper warned this was a concerning development after previous rhetoric Putin has spouted in recent days.
"This is something that concerns me on the heels of his previous warning," he said.
"I'll quote: 'Whoever tried to interfere should know that Russia's response will be immediate and will lead to such consequences that you have never experienced in history'."
"I personally think he's unhinged," he said, adding: "I really worry about his acuity and balance right now.
"Here is a guy who really has his finger on a nuclear button. To me it bears close watching".
Former deputy director of national intelligence Beth Sanner was more reserved in her evaluation of events, saying Putin was doing this "out of weakness", and added: "It's scary but he doesn't have much else."
Sanner added: "We have said as an intelligence community all along that Putin really wants to avoid a nuclear war with the United States because obviously that would be bad."
Nuclear exchange 'unthinkable' for Moscow
Dr Thomas Wilkins, senior lecturer in international security from the University of Sydney's Department of Government and International Relations, says both Russia and the US have a "constant readiness" when it came to their nuclear assets.
"The question is if he is putting more forces on alert, which is a clear escalation," he told Yahoo News Australia.
"I'm inclined to believe it's for theatrical effect to "deter" further "hostile" moves by Western allies.
"Putin may have gambled with the invasion of Ukraine, but it's unthinkable that Moscow would want this to end in a nuclear exchange."
He said the subjugation of Ukraine "could never be worth a nuclear conflict to Russia".
Putin's nuclear threat is 'proving a point'
Curtin University political analyst Professor Joe Siracusa said the announcement was an indication of how quickly Russia can escalate their attack.
"Just to prove the point how dangerous he is – he’s put his nuclear weapons on alert," he told Sky News Australia.
“What that means is – they’re on a 15 minute alert. [Russia] only needs 15 minutes to get these things going in the direction [Putin] wants them to go.”
Additional reporting by Tom Flanagan