US President Joe Biden has branded his Russian counterpart Vladimir Putin a "war criminal" over his invasion of neighbouring Ukraine.
"I think he is a war criminal," Biden said in an off-the-cuff response to a reporter's question.
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 through his invasion of a foreign country".
It's the sharpest condemnation yet of Putin and Russian actions by a US official since the invasion of Ukraine.
While other world leaders have used the words, the White House had been hesitant to declare Putin's actions those of a war criminal, saying it was a legal term that required research.
The Kremlin has called the labelling "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," Kremlin spokesman Dmitry Peskov told Russian state news agency Tass.
The US President had earlier signed off a €723 million security aid package, bringing the US’s total contribution to more than one billion.
The package is a key element in helping Ukraine get a long-range air defence system and access to drones.
However, Ukrainian President Volodymyr Zelenskyy said it was still not enough.
"To establish a no-fly zone over Ukraine in order to save people, is it too much to ask? A humanitarian no-fly zone is a condition under which Russia will no longer be able to terrorize our peaceful cities every day and night," said Zelenskyy.
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.
Ukraine's military, backed by a heavy flow of Western weapons, has fought back, largely stalling the Russian advance.
Russian troops have turned increasingly to bombardments of civilians, prompting three million Ukrainians to become refugees.
In Moscow, President Vladimir Putin delivered a stark warning to so-called Russian "traitors", who he said the West wanted to use as a "fifth column" to destroy the country.
"The collective West is trying to split our society - speculating on military losses, on socioeconomic consequences of sanctions - to provoke a civil confrontation in Russia and, using its fifth column, strives to achieve its goal. And the goal is one, I have already said this, the destruction of Russia," he said.
Russia experts fear the message could lead to a new round of repression against those who don't agree with the regime.