US President Joe Biden has called his Russian counterpart Vladimir Putin a "crazy son of a bitch" during a fundraising event in San Francisco on Wednesday.

"We have a crazy S.O.B. that guy, Putin, others. And we always have to be worried about a nuclear conflict. But the existential threat to humanity is climate," CNN quotes Biden.

Biden also criticised comments made by former president Donald Trump, who compared his legal problems to the death of Alexei Navalny, who is considered to be a Russian opposition leader and Kremlin critic. Biden paraphrased his predecessor's comments and then told the audience that "if I stood here 10 to 15 years ago and said all this, you’d all think I should be committed."

"He’s comparing himself to Navalny and saying that because our country’s become a communist country, he was persecuted, just like Navalny was persecuted. Where the hell does this come from," Biden added. 

Unlike Biden and other Western leaders, Trump, the Republican frontrunner in the 2024 presidential election, has yet to condemn Russia or Putin for Navalny's death.

At a campaign event, Donald Trump described how he had threatened to "encourage Russia" to act aggressively against the allies in order to force them to increase their contributions to NATO's collective security.

His words caused a huge outcry among the allies.  

The NATO Secretary General asked Trump not to undermine the deterrence system.