Biden seems to insist on an escalation of words against his Russian counterpart, as, the first day, he renewed his attack on Putin, calling him a "murderous dictator" and a "pure serial killer."

The Russian Foreign Ministry said it had summoned US Ambassador John Sullivan to tell him that US President Joe Biden's statements about his Russian counterpart Vladimir Putin had pushed relations between the two countries to the brink of collapse.

The Russian Foreign Ministry, in a statement published after the U.S. ambassador's summons, considered that "this kind of statement by an American president that is not appropriate for a first-row statesman has brought Russian-American relations to the brink of ruin."

According to the statement, US Ambassador John Salevan delivered an official protest letter regarding "unacceptable statements" made recently by Biden.

The Ministry noted that it had warned the Ambassador that "hostile actions against Russia will receive a firm response".

Last week, in response to a question from reporters, Biden said he believed Putin was a war criminal, which angered the Kremlin.

Kremlin spokesman Dmitry Peskov said at the time that his country considered the speech "unacceptable and unforgivable, to a head of State whose bombs killed hundreds of thousands of people around the world," according to TAS.

The White House spokeswoman tried to soften her President's statements, saying that he was speaking from the heart after witnessing the devastation of the war in Ukraine.

Ukraine and its Western allies accuse Moscow of attacking civilians without discrimination, while Russia describes its military actions in Ukraine as a "special operation," and denies attacking civilians.