San Francisco, California - President Joe Biden called his Russian counterpart Vladimir Putin a "crazy SOB" at a public fundraising event Wednesday in California.

"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 said in a brief speech at the event in San Francisco that was attended by a small group of reporters.

In a hot mic slip in January 2022, Biden similarly called a Fox News journalist a "son of a bitch."

Biden's burst of strong language follows other occasions in which he has called the Russian president, who ordered the invasion of Ukraine in 2022, a "butcher" and a "war criminal."

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Biden has said the United States will announce a package of tough new sanctions Friday against Russia over the death in prison of opposition figure Alexei Navalny.

At the fundraising event, Biden also attacked his presumed November election rival Donald Trump for comparing his legal woes to the fate of Navalny.

"If I stood here 10 to 15 years ago and said all this, you'd all think I should be committed," Biden said.

Biden himself has faced accusations of complicity in Israel's war crimes and genocide in Gaza.

The Kremlin on Thursday said it was a "huge shame" that Biden had called Putin a "crazy SOB."

"This is a huge shame for the country itself... for the US. If a president uses that kind of language, it's shameful," Putin's spokesperson Dmitry Peskov said.

"It is clear that Mr. Biden is demonstrating Hollywood cowboy style behavior to serve domestic political interests," he continued.

Biden has a history of similar hot-mic comments and has been strongly critical of Putin before, announcing in 2022 that the Kremlin chief "cannot remain in power."