The European Union presidency has called for an international tribunal over Russia's invasion of Ukraine.
The call, from the Czech Republic which currently holds the rotating presidency of the bloc, came after the discovery of hundreds of graves in Izyum, a town recently liberated by Ukrainian troops.
Many are said to be civilians, women and children among them.
"We stand for the punishment of all war criminals," Czech Foreign Minister Jan Lipavsky said.
Ukraine says it believes war crimes have been committed in Izyum, where 59 bodies have been exhumed so far – with more expected from the graves in a forest at the edge of the city.
"In the 21st Century, such attacks against the civilian population are unthinkable and abhorrent," Mr Lipavsky said.
"We must not overlook it. We stand for the punishment of all war criminals," he said.
"I call for the speedy establishment of a special international tribunal that will prosecute the crime of aggression."
In his regular address on Saturday evening, Ukrainian President Volodymyr Zelensky said investigators had discovered new evidence of torture used against the people buried in Izyum, in Kharkiv region.
"More than 10 torture chambers have already been found in the liberated areas of Kharkiv region, in various cities and towns," Mr Zelensky said.
He said the Russians would have to answer "both on the battlefield and in courtrooms".
On Thursday, EU Commission President chief Ursula von der Leyen said she wanted Mr Putin to face the International Criminal Court over war crimes in Ukraine.
Russia claims it is fighting to de-Nazify Ukraine, in a conflict it still refers to as a "special military operation" rather than a war.
It has not commented on the burial sites at Izyum. Moscow has previously denied targeting civilians.
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Watch: Putin failing on all his strategic military objectives – Radakin
The discovery of the burial sites came as Ukrainian troops continue their counter-offensive in the country's north-east, after successfully recapturing territory from Russia in recent days.
Russian President Vladimir Putin said on Friday that Ukrainian counter-offensives would not change Russia's military plans in the east of Ukraine.
The UK defence ministry says Russia is intensifying attacks on civilian infrastructure in Ukraine.
"As it faces setbacks on the front lines, Russia has likely extended the locations it is prepared to strike in an attempt to directly undermine the morale of the Ukrainian people and government," it said in its daily assessment of the conflict.
But Mr Putin is failing "on all his military and strategic objectives" in Ukraine, UK Chief of the Defence Staff Admiral Sir Tony Radakin told the BBC on Sunday.
Adm Radakin, however, urged caution as "the likely result with all of this is that it's going to grind on for a long time.
"And that's why there's a wishfulness when people jump to conclusions that either President Putin is weak and his power base might be undermined, or that Ukraine has gained some ground and there's been a magnificent action in the north-east.
"But it doesn't automatically lead on to easy victories elsewhere."
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