The U.S. president made the "crazy SOB" remark as part of a sentence about threats to the world - including "that guy Putin and others", the risk of nuclear conflict and the existential threat to humanity from climate change.

"The use of such language against the head of another state by the president of the United States is unlikely to infringe on our president, President Putin," Kremlin spokesman Dmitry Peskov told Reuters. "But it debases those who use such vocabulary."

Peskov said the remark was "probably some kind of attempt to look like a Hollywood cowboy. But honestly I don't think it's possible."

"Has Mr Putin ever used one crude word to address you? This has never happened. Therefore, I think that such vocabulary debases America itself," Peskov said, adding that such language was a disgrace for the United States.

Putin ally Dmitry Medvedev, who served as president from 2008-2012, said the existential threat to the world came from "useless old geezers, like Biden himself". Medvedev said Biden was "senile" and "ready to start a war with Russia".

The war in Ukraine, the death of Russian opposition leader Alexei Navalny and U.S. assertions that Russia plans to put a nuclear weapon in space have led to the biggest crisis in relations between Russia and the West since the Cold War.

Some Russian and U.S. diplomats say they do not remember a time when relations between the world's two biggest nuclear powers were worse, including during the 1962 Cuban Missile Crisis.

Biden said last week after prison officers announced Navalny's death in a Russian penal colony that it was "a consequence of something that Putin and his thugs did." Navalny had previously accused Putin of trying to kill him, an allegation the Kremlin denied.

Russian officials say the West rushed to blame Putin without waiting for evidence. The Kremlin says the West's reaction to Navalny's death is unacceptable and unjustified.

Biden said in a speech in Warsaw in 2022 that Putin "cannot remain in power". The White House played down the remark, while hardliners in Russia saw it as evidence that the U.S. wanted to topple Putin.

In 2021, Biden said he thought Putin a killer. Putin said Biden phoned him later to give an explanation of why he used such words.

(Reporting by Guy Faulconbridge, Editing by Timothy Heritage and Hugh Lawson)

By Guy Faulconbridge