President Joe Biden called Russian President Vladimir Putin a "crazy SOB" during a fundraiser in San Francisco on Wednesday, warning there is always the threat of nuclear conflict but that the existential threat to humanity remains climate, Paralel.Az reports citing Reuters.

"This is the last existential threat. It is climate. We have a crazy SOB like that guy Putin and others and we always have to worry about nuclear conflict, but the existential threat to humanity is climate," Biden told a small group of donors.

Biden has previously cursed "son of a bitch" at others. In January 2022, he was caught on the hot mic using the same term of abuse against a Fox News White House reporter.

Biden has a tendency to go off script during election fundraisers and in recent months has dug into the Chinese government, the Republican Party and U.S. ally Israel for its bombing of the Gaza Strip.

Biden's verbal attacks against Putin have also sharply intensified at the White House and on the campaign trail. Last week, the U.S. President blamed Putin and "his thugs" for the death of opposition leader Alexei Navalny.

"We don't know exactly what happened, but there is no doubt that the death of Nalvany was a consequence of something that Putin and his thugs did," Biden said at the White House after Russian prison officials announced that Navalny had died.

The Kremlin has denied involvement in Navalny's death and said Western claims that Putin was responsible are unacceptable.

Biden and Putin remain deeply at odds over Russia's invasion of Ukraine two years ago, over which Russia has been sanctioned by the United States and other Western nations. Biden's reactions have put a further chill into already bitter U.S.-Russian relations.

On Tuesday, Biden said the U.S. will announce a major package of sanctions against Russia over Navalny's death and the Ukraine war.

Biden's expected Republican opponent in November, former President Donald Trump, has expressed admiration for Putin both during his 2017-2021 White House tenure and afterward. However, he also recently compared himself to Navalny, implying that they both had faced politically motivated prosecutions.

"I don't know where the hell this comes from," Biden said on Wednesday reacting to Trump comparing himself to Navalny.