The US has “formally determined” that Russia had committed crimes against humanity in Ukraine, US Vice-President Kamala Harris has said.
Speaking at the Munich Security Conference, Ms Harris accused Russia of “gruesome acts of murder, torture, rape and deportation” since its invasion.
Moscow’s ambassador to the US rejected the claims and accused Ms Harris of trying to “demonise Russia”.
World leaders at the conference called for long-term support of Ukraine.
UK PM Rishi Sunak said now was the time to “double down” on military support.
The prime minister argued that Western allies must start planning for the future security of Ukraine, as well as sending the weapons it needs to defend itself now.
The conference in Germany comes as the one-year anniversary of Russia’s full-scale invasion of Ukraine approaches on 24 February.
Ms Harris told delegates that the perpetrators of alleged Russian crimes in Ukraine must be held to account.
“Their actions are an assault on our common values and our common humanity,” she said.
The UN defines crimes against humanity as a “widespread or systemic attack” on a particular civilian population.
Moscow has repeatedly denied targeting civilians during its invasion.
Anatoly Antonov, Russia’s ambassador to the US, said the vice-president’s claims were a cynical attempt to “demonise Russia in the course of a hybrid war”.
They were a way of “justifying Washington’s own actions to fuel the Ukrainian crisis”, he added, referring to the US supply of arms to Kyiv.
But Ms Harris, a former prosecutor, was adamant that “in the case of Russia’s actions in Ukraine we have examined the evidence, we know the legal standards, and there is no doubt: these are crimes against humanity”.
She cited “barbaric and inhumane” atrocities committed during the war in Ukraine, including the scores of bodies found in Bucha shortly after the invasion and the bombing of a theatre in Mariupol.
“Let us all agree: on behalf of all the victims, both known and unknown, justice must be served,” Ms Harris said.
Crimes against humanity are tried at the International Criminal Court (ICC).
But the ICC has no powers to arrest suspects and can only exercise jurisdiction within countries which signed up to the agreement that set up the court.
Russia is not a signatory to that agreement, so it is unlikely to extradite any suspects.
The three-day gathering in Munich will provide a key test of Western support for Kyiv as both sides in the war prepare for spring offensives.
Ukraine’s Foreign Minister Dmytro Kuleba said Moscow had “waged a genocidal war” because it did not think Ukrainians “deserve to exist as a sovereign nation”.
Tens of thousands have lost their lives and millions have been forced from their homes as part of Vladimir Putin’s invasion.
This conference has largely been a gathering of American and European leaders. It’s a chance for them to reaffirm their support for Ukraine and demonstrate their resolve.
Rishi Sunak called for a new Nato charter to guarantee Ukraine’s long-term security. Kamala Harris formally accused Russia of committing crimes against humanity.
But in the margins, there have been voices of doubt.
Take the prime minister of Namibia, Saara Kuugongelwa-Amadhila. She opposed sending more arms to Ukraine and called for a peaceful resolution to the war. Her country, she said, had suffered recession, rising prices and disrupted supply chains.
It’s opinions like that, widely held cross Africa, Asia and South America, that are concentrating transatlantic minds.
There is realisation among Western policymakers that almost one year after Russia’s invasion, they need to remake the case for defending Ukraine.
Russia was also on the agenda during a meeting on Saturday between the Antony Blinken and Wang Yi, the US and China’s top foreign policy officials.
During talks at the conference in Munich, Mr Blinken warned of consequences if China were to provide material support to Russia’s invasion.
Mr Blinken is expected to suggest China is “at least contemplating providing” lethal assistance to Russia in an interview to be aired on Sunday morning on NBC News.