Russian leader Vladimir Putin should stand trial for war crimes, US president Joe Biden has said.
Speaking to reporters on Monday in Washington, the president also indicated that the United State was "seeking more sanctions" to be applied against Russia on foot of reported atrocities in Ukraine.
Mr Biden said he had been criticised previously for calling Mr Putin a war criminal.
"Well, the truth of the matter – you saw what happened in Bucha. He is a war criminal," he said.
“We have to continue to provide Ukraine with the weapons they need to continue the fight. And we have to gather all the detail so this can be an actual – have a war crimes trial.”
The president hit out at Mr Putin as “brutal”.
“What’s happening in Bucha is outrageous and everyone’s seen it,” Mr Biden said.
However, Mr Biden did not think the actions of the Russians in Bucha amounted to genocide. “No, I think it is a war crime,” he said.
The Kremlin has categorically denied any accusations related to the murder of civilians in the town and said the graves and corpses had been staged by Ukraine to tarnish Russia.
US national security adviser Jake Sullivan said there had been atrocities and war crimes but the United States had not yet seen evidence where actions by the Russians had risen to the level of genocide. He indicated that the US would consult with its allies about the forum for dealing with war crimes in Ukraine.
Mr Sullivan said the US would announce new sanctions against Russia later this week.
The White House suggested that Russia would now intensify its campaign in eastern Ukraine with probable plans to "deploy tens of thousands of soldiers" to that region.
On a visit to Bucha, about 30km north-west of the Ukrainian capital, Kyiv, the Ukrainian president, Voloymir Zelenskiy, maintained that the town had been the scene of war crimes that would be "recognised by the world as genocide".
“You stand here today and see what happened. We know thousands of people have been killed and tortured with extremities cut off; women raped, children killed.”
President Biden said Putin “should be held accountable”.
He said he was going to continue to add to the sanctions being imposed on Russia for its invasion of Ukraine but declined to go into details.
The US military indicated that it was not in a position to independently confirm Ukrainian accounts of atrocities by Russian forces against civilians in Bucha. However, a senior US defence official said on Monday that it had no reason to dispute the accounts either.
“We’re seeing the same imagery that you are. We have no reason whatsoever to refute the Ukrainian claims about these atrocities – clearly deeply, deeply troubling,” the official said.
European Union officials separately said on Monday they were preparing additional sanctions to penalise Russia. French president Emmanuel Macron called for "very clear measures" and said the penalties would be discussed over the next few days.
Germany announced on Monday it would expel 40 staff members of the Russian embassy in Berlin with suspected links to Russian spy agencies as a first response to the alleged killing of civilians. Foreign minister Annalena Baerbock said in a statement that images of the victims show "the unbelievable brutality of the Russian leadership" and those who follow its propaganda.
France is also to expel Russian diplomatic staff, with French news agency AFP reporting that 35 people would be asked to leave the country.