US President Joe Biden called Russia’s President Vladimir Putin a “war criminal” on Wednesday as Moscow intensifies its attacks on Ukraine, to which the Kremlin responded by saying that Biden’s comments were “unacceptable and unforgivable”.

Biden told reporters in a press conference: “I think he is a war criminal”.

This marked a change in Washington’s rhetoric which condemned the atrocities committed by Russia in Ukraine but stopped short of labelling them “war crimes”, citing ongoing investigations by international organizations.

White House Press Secretary Jen Psaki said in a briefing: “I think he is a war criminal,” adding that Biden was “speaking from the heart.”

“[Biden] was answering a direct question that was asked and responding to what he has seen on television. We have all seen barbaric acts, horrific acts by a foreign dictator in a country that is threatening and taking the lives of civilians — impacting hospitals, women who are pregnant, journalists, others and I think he was answering a direct question,” she added.

Kremlin Spokesman Dmitry Peskov told reporters Biden’s description of Putin was “unacceptable and unforgivable rhetoric”.

“We believe such rhetoric to be unacceptable and unforgivable on the part of the head of a state, whose bombs have killed hundreds of thousands of people around the world,” Russian state news agency TASS cited Peskov as saying.

Biden’s statement regarding Putin came after Ukrainian President Volodymyr Zelenskyy delivered an impassioned video address to US Congress, asking for more support for his country which is struggling to hold off the Russian onslaught.

The US President announced an additional $800 million in security assistance to Ukraine, including anti-aircraft systems and drones.