As of now more than three million people have fled Ukraine since Russian invasion.
President Joe Biden labelled Vladimir Putin a "war criminal" on Wednesday after watching harrowing TV footage of the invasion of Ukraine, triggering an immediate angry riposte from Russia.
It was the first time Biden has used the phrase to describe Putin, and the US president then deepened his attack, tweeting that Putin was "inflicting appalling devastation and horror on Ukraine -- bombing apartment buildings and maternity wards."
Referring to reports that Russian troops took hostage doctors and patients in a Mariupol hospital, Biden said that "these are atrocities. It is an outrage to the world."
White House Press Secretary Jen Psaki said Biden was "speaking from his heart" after seeing images on television of "barbaric actions by a brutal dictator."
Putin's spokesman Dmitry Peskov told Russian news agencies that Biden's rhetorical attack was "unacceptable and unforgivable on the part of the head of a state, whose bombs have killed hundreds of thousands of people around the world."
Putin ordered a large-scale invasion of Ukraine three weeks ago, saying Russia wants to force the disarmament of Ukraine's military and topple the pro-Western government.
With the Russian advance largely stalled by Ukrainian troops, Moscow has turned increasingly to bombardments of civilians, prompting three million Ukrainians to become refugees.
Psaki said the US State Department already has "a legal process that is ongoing" to examine Russia's actions.
Ukrainian President Volodymyr Zelensky made an impassioned video address to US lawmakers earlier Wednesday, bolstered by a montage of horrific TV footage of Ukrainian civilians under Russian assault.
Last year, Russia briefly recalled its US ambassador after Biden likened Putin to a "killer."
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