US President Joe Biden called his Russian counterpart Vladimir Putin a "crazy SOB" at a public fundraising event on Wednesday, which the Kremlin decried as "shameful."

The 81-year-old leader has regularly had harsh words for Putin since Moscow's assault on neighbouring Ukraine two years ago, including calling him a "butcher" and a "war criminal."

And on Wednesday he was no less direct.

"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 attended by a small group of reporters.

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

Putin's spokesman Dmitry Peskov said Biden's invective was "a huge shame for the country itself... for the US."

"If a president uses that kind of language, it's shameful," he said.

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

In an interview with ABC News in March 2021, Biden was asked if he thought Putin was "a killer". He replied: "I do."

Moscow recalled its ambassador to Washington for consultations, and Putin retorted that it "takes one to know one."

In a speech from the Oval Office on October 20, 2023, Biden compared Palestinian militant group Hamas and Putin, saying they "both want to completely annihilate a neighbouring democracy."

"We cannot and will not let terrorists like Hamas and tyrants like Putin win. I refuse to let that happen," he said.

The Kremlin denounced the remarks as "unacceptable."

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. (AFP)