Ron Klain says Putin using a nuclear weapon is 'not impossible to imagine' and Biden is preparing for the possibility, book

• Biden is preparing in case Putin decides to use nuclear weapons, his chief of staff said in October.
• Biden is "'very focused on being prepared in case Putin does it, and trying to figure out what we can do to deter him,'" Ron Klain said.

President Joe Biden has said he takes seriously the threat of Russian President Vladimir Putin using nuclear weapons, and he's preparing for it behind the scenes.

White House Chief of Staff Ron Klain told author Chris Whipple, in an October interview, that the prospect isn't outside the realm of possibility.

"'We're on a path that could lead to that outcome. We don't know what Putin will do, but tragically, it's not impossible to imagine,'" Klain is quoted as saying in Whipple's book, "The Fight of His Life: Inside Joe Biden's White House," released on January 17.

Klain told Whipple at the time that Biden is "'very focused on being prepared in case Putin does it, and trying to figure out what we can do to deter him from doing it.'"

Whipple conducted the interview with Klain on October 8, a couple of days after Biden spoke about Putin at a fundraiser in New York City and raised the prospect of "Armageddon."

Biden suggested at the time that Putin was "not joking" about potentially using tactical nuclear weapons or chemical weapons because his military was underperforming in Ukraine, according to Wall Street Journal reporter Tarini Parti and The Associated Press.

"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis," the president said at the time.

Whipple wrote that Biden "was preoccupied with the possibility that Putin might use a nuclear weapon" and shared CIA director Bill Burns' "belief that, in the Russian autocrat's calculus, the stakes in Ukraine were existential."

Putin has repeatedly made nuclear threats since launching the unprovoked, full-scale invasion of Ukraine last February, including as he escalated the war in September by announcing a partial military mobilization.

"Those who try to blackmail us with nuclear weapons should know that the prevailing winds can turn in their direction," Putin said at the time.

Burns, a former US ambassador to Russia, told Whipple in April that the CIA had done "a variety of assessments" on the likelihood of Putin ordering a nuclear strike. "'Given the kind of saber rattling that we've heard from the Russian leadership, we can't take lightly those possibilities,'" he said, according to the book.

Analysts raised alarms at Putin's statements, but maintained the chances of him actually using such a weapon still remain low.

Many top Russia experts have suggested that Putin's nuclear threats are largely designed to deter the West from continuing to provide Kyiv with security assistance. The US and its allies have provided Ukraine with billions in aid since the war began, including vital weaponry that helped Ukrainian forces repel the Russian invaders and make significant progress in a counteroffensive launched in August.

Indeed, Western countries have not allowed Putin's threats to deter them from providing Ukraine with weapons, but with ongoing questions and concerns about the Kremlin's red lines they've also been cautious about sending certain types of arms and equipment. The issue has come up again in recent discussions surrounding a Western impasse on providing battle tanks to Ukraine. During a panel at the World Economic Forum in Davos this week, former UK Prime Minister Boris Johnson said that the West should not allow the fear of provoking Putin to prevent it from supplying heavy weapons to Ukraine.

"He's not going to use nuclear weapons," Johnson said of Putin, per The Guardian, making the case that doing so would put Russia in a "cryogenic paralysis" and turn countries like India and China against Moscow.

"People argue that we should not escalate and should be wary of doing things that further provoke Putin," Johnson said. "How can we escalate against a guy who is doing all-out war against a civilian population?"

Insider's Kelsey Vlamis and Lloyd Lee contributed to this story.