Vladimir Putin "cannot remain in power" as he tries to "rebuild an empire", Joe Biden has declared.
The US President blasted Putin as a "dictator" in a stirring speech at the Royal Castle in Warsaw, Poland.
It comes after years of claims that Putin is desperate to re-establish Russian control over former Soviet Union countries.
Biden denounced the despot after speaking to Ukrainian refugees in the Polish capital today.
When asked what he thought of Putin after the meeting, the President said: "He's a butcher."
In his speech outside the Royal Castle, Biden added: "In this hour, let the words of Pope John Paul burn as brightly today - never, ever give up hope, never doubt, never tire, never become discouraged. Be not afraid.
"A dictator bent on rebuilding an empire will never erase a people's love for liberty.
"Brutality will never grind down their will to be free. Ukraine will never be a victory for Russia, for free people refused to live in a world of hopelessness and darkness.
"We will have a different future, a brighter future, rooted in democracy and principle, hope and light, of decency and dignity, of freedom and possibilities.
"For God's sake, this man cannot remain in power."
The White House later clarified that Biden was not discussing regime change in Moscow.
Instead he was emphasising that Russia should not be allowed to control its neighbours, a US official said.
Biden also addressed the Russian people in his speech today, saying 'you are not our enemy' while warning that the Kremlin's actions in Ukraine was 'not (those) of a great nation'
Biden added: "Of all people, you the Russian people, as well as all people across Europe still have the memory of being in a similar situation in the 30s and 40s, the situation of World War Two, still fresh in the mind of many grandparents in the region."
After sharing accounts of the horrors of World War Two, he added: "These are not memories of the past - not any more, it's exactly what the Russian army is doing in Ukraine right now."
The US president said Moscow's troops have "met their match with brave and stiff Ukrainian resistance" and have strengthened the resolve and unity of both the defending forces and the West.
"Russia wanted less of a Nato presence on his border but now he has a stronger presence, a larger presence," he said.
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Mr Biden said 200,000 Russians had left their country in a single month in a "remarkable brain drain" as Mr Putin, who he earlier called a "butcher", had "strangled democracy".
Addressing the thousand-strong audience that included refugees who have fled the war, he told Ukrainians "we stand with you - period" as he defended the nation's president, Volodymyr Zelensky.
"Putin has the gall to say he's denazifying Ukraine. It's a lie, it's just cynical - he knows that," Mr Biden said.
"And it's also obscene. President Zelensky was democratically elected, he's Jewish, his father's family was wiped out in the Nazi holocaust and Putin has the audacity - like all autocrats before him - to believe that might will make right."
