A top Russian official repeated Russia's nuclear threats, saying they're not a bluff.
Dmitry Medvedev said NATO countries wouldn't step in if Russia fired a nuke at Ukraine.
One expert told Insider it likely was a bluff — but that it should be taken seriously anyway.
Russia's former president repeated the country's threat to use nuclear weapons in Ukraine and mocked NATO by saying it would not come to Ukraine's aid if Russia struck.
On Telegram Tuesday, Dmitry Medvedev, who is now Russia's Security Council chief, criticized US President Joe Biden, UK Prime Minister Liz Truss, and the wider NATO alliance.
He said those leaders "constantly threaten us with 'terrifying consequences' if Russia uses nuclear weapons" and accused Truss of being "completely ready to immediately begin an exchange of nuclear strikes with our country."
Medvedev said Russia's laws around the use of nuclear weapons meant it could retaliate with them if it's hit with nukes or attacked with conventional weapons that threatened "the very existence of our state."
Russia will also "do anything" to prevent the nuclear weapons emerging in the country's "hostile neighbors," such as Ukraine, Medvedev said.
"If the threat to Russia exceeds the established danger limit, we will have to respond," he said. "Without asking anyone's permission, without long consultations. And it's definitely not a bluff."
Medvedev said that if Russia did strike Ukraine with a nuclear weapon, NATO member states would put their own security ahead of protecting "a dying Ukraine that no one needs."
An isolated Russia
Michael Clarke, a professor and security expert, told Insider he believed Putin using a nuclear weapon "would bring him down immediately."
"The whole world would turn against him," he said, adding that he believed NATO would hit back, albeit with conventional weapons. And China would likely drop its muted support for Russia, he said.
Clarke, the associate director of the Strategy and Security Institute at the UK's University of Exeter, said: "I think there would be an immediate upping of the campaign, and I suspect that would actually bring Western forces into Ukraine."
Medvedev's remarks come as Russia moves to annex large parts of occupied Ukraine through sham referendums.
Should President Vladimir Putin announce annexations — expected this Friday, according to the UK's Ministry of Defense — any attempt to retake those areas may be interpreted in Moscow as an attack on Russia itself.
But a recent spate of statements like these is "trying to ramp up the threat" and scare the West away from further support of Ukraine because Putin is "in a corner," Clarke said.
Putin's recent announcement calling up reservists has been viewed internationally as a desperate act spurred by Ukraine's successful counterattack.
He was recently snubbed even by semiallies such as Turkey, India, and China.
Clarke said Putin was humiliated.
Is Putin bluffing?
After Putin's latest statement, the White House said Russia would face "catastrophic consequences" if it used tactical nuclear weapons.
In an interview aired Sunday, Ukrainian President Volodymyr Zelenskyy said he didn't think Putin was bluffing.
Clarke said he believed Russia's threats were indeed a bluff — but that the West must still take them seriously.
"Even when people are bluffing, they find themselves running out of room to maneuver and then get locked into doing what they thought they wouldn't have to do," he said.
He added: "Whenever you start to play games with nuclear strategy, the danger of a mistake or sheer miscalculation can never be ruled out. So yes, the West has got to take it seriously."
The defense minister of Russia's neighbor Latvia also told Insider he believed Putin was likely bluffing in hopes of getting the West to reduce its support for Ukraine.
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