Ukrainian authorities have found a mass grave in the recaptured city of Izium, President Volodymyr Zelensky has said.
In a video address, he said that more information should be available tomorrow.
"Russia is leaving death behind it everywhere and must be held responsible," he said.
Meanwhile, EU chief Ursula von der Leyen has said that she wants Russian President Vladimir Putin to face the International Criminal Court over war crimes in Ukraine.
"That Putin must lose this war and must face up to his actions, that is important to me," she told the TV channel of German news outlet Bild.
There was no doubt that war crimes had been committed in Ukraine, said Ms von der Leyen, who also visited Kyiv today.
"Therefore we support the collection of evidence" with a view to possible proceedings at the International Criminal Court, she said.
"That is the basis of our international legal system, that we punish these crimes. And ultimately, Putin is responsible," she said.
Asked whether Mr Putin would one day be brought before the court, she responded: "I believe it is possible."
During her visit to Ukraine for talks on closer integration with the European Union, Ms von der Leyen pledged that the war-torn country would have Europe's support for "as long as it takes".
Her trip coincided with a meeting between Russian President Vladimir Putin and his increasingly close ally Chinese leader Xi Jinping in ex-Soviet Uzbekistan, where the men hailed their strategic ties in defiance of the West.
It also comes as Ukrainian forces consolidate gains against Russian forces in the east of the country in a dramatic offensive aided by Western weapons.
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Ms Von der Leyen said that today's visit was her third to Ukraine since Russia invaded in February, but this one was different.
"We will never be able to match the sacrifice that the Ukrainians are making," she told reporters during a joint press conference with Mr Zelensky.
"But what we can tell you is that you'll have your European friends by your side as long as it takes," she said.
Upon her arrival she noted her discussions with Ukraine's leaders would be about "getting our economies and people closer".
Ukraine gained EU candidacy status in June at the same time as ex-Soviet Moldova, which borders Ukraine and like its neighbour has had Russian troops stationed in an eastern breakaway region.
Mr Zelensky told the same press conference that his country wants to join the European single market ahead of a decision on whether to grant Kyiv full EU membership.
Zelensky promises 'victory'
The historic candidacy vote angered Moscow, which has tried to retain political and military influence in both countries since the collapse of the Soviet Union three decades ago.
EU countries have staunchly supported Ukraine since Moscow invaded in February by hitting Russia with economic penalties.
Many members of the bloc have supplied Kyiv with advanced weapons that have helped Ukrainian forces in recent weeks recapture swathes of territory.
Ms Von der Leyen said just ahead of her trip that the successive waves of EU sanctions against Russia would remain and that Europeans must keep their resolve against Moscow.
"I want to make it very clear, the sanctions are here to stay," she told the European Parliament, during her State of the Union address yesterday.
The Kremlin maintains that Russia has weathered the economic penalties and Moscow has responded by reducing or halting entire gas flows to European countries.
With winter fast approaching, this has forced the EU to source alternative supplies, agree plans to cut consumption and roll out financial support in the face of skyrocketing prices.
Ukrainian energy facilities, including Europe's biggest atomic plant, have been hit by Russian strikes and Mr Zelensky today thanked the EU leader for linking Kyiv to "European power grids".
"It's in everyone's interest: European countries can get cheap electricity from Ukraine" and "we'll be able to get money for salaries and social payments in such difficult times," he said.
Military situation remains 'extremely difficult'
Ukraine's forces have also posted slower, but steady gains in the southern Kherson region near the Black Sea.
The Ukrainian presidency said that intense fighting was ongoing around that southern front, adding that the military situation "remains extremely difficult."
In the eastern Donetsk region, which has been partially controlled by Russian-backed separatists since 2014, fresh shelling killed two civilians and left another 13 wounded.
In the Ukrainian-held Donetsk town of Bakhmut, AFP journalists said streets were deserted and the sound of nearby artillery was audible in the town's centre.
Flames were still licking a residential building hit by Russian strikes overnight and plumes of thick smoke were rising as firefighters battled the blaze.
The Ukrainian government has also condemned a Russian missile strike on a dam near the central city of Kryvyi Rih, describing it as an act of revenge for Ukraine's counter-offensive in the northeast.
Officials say the dam was breached, affecting water supplies and risking serious flooding.
President Zelensky said the dam was of no military value and that hundreds of thousands of people depended on it.
Kryvyi Rih, the largest city in central Ukraine and Mr Zelensky's home city, was targeted by eight cruise missiles yesterday, officials said.
The city had an estimated pre-war population of 650,000.
"The water pumping station was destroyed. The river broke through the dam and overflowed its banks. Residential buildings are just a few metres away from the river," Ukrainian legislator Inna Sovsun said on Twitter.
The centre and another district of the city were at risk of flooding, said Kyrylo Tymoshenko, deputy head of the president's office.
Ukraine and the West accuse Russia of an unprovoked war of aggression.
In a move that suggests Mr Putin had wider war aims when he ordered troops into Ukraine on 24 February, three people close to the Russian leadership told Reuters that Mr Putin had rejected a provisional deal with Kyiv around the time the war began.
They said the deal would have satisfied Russia's demand that Ukraine stay out of the US-led Western military alliance NATO.
The Kremlin said the Reuters report had "absolutely no relation to reality". It also said Ukraine's ambitions to join NATO still presented a threat to Russia.
