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.Media caption,
Evidence grows of civilian killings in Bucha
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.