US President Joe Biden believes that Russia will not use nuclear weapons in a war with Ukraine.

"I don't think he [Putin] will," Biden said when asked whether the Russian leader would use a tactical nuclear weapon.

"I think it's irresponsible for him to talk about it, the idea that a world leader of one of the largest nuclear powers in the world says he may use a tactical nuclear weapon in Ukraine," Biden added.

Biden said even Putin's threats have a destabilizing effect, and warned of the potential errors in judgment that could ensue.

"The whole point I was making was it could lead to just a horrible outcome," he said. "And not because anybody intends to turn it into a world war or anything, but just once you use a nuclear weapon, the mistakes that can be made, the miscalculations, who knows what would happen."

"He, in fact, cannot continue with impunity to talk about the use of a tactical nuclear weapon as if that's a rational thing to do," Biden added later. "The mistakes get made. And the miscalculation could occur, no one can be sure what would happen and could end in Armageddon."

Biden refused to disclose what a US response would look like should Putin follow through on his nuclear threats. But he said the Department of Defense had proactively developed contingencies should the scenario come to pass. "There's been discussions of that, but I'm not going to get into that. It would be irresponsible of me to talk about what we would or wouldn't do," Biden said.