Ukraine says it has evidence of a mass grave in Izyum, the north-eastern city recently freed from Russian occupation.
Regional police head Volodymyr Tymoshko told the BBC more than 400 bodies were thought to have been buried there.
“Russia leaves death everywhere,” President Volodymyr Zelensky said, mentioning other reported mass graves found after Russian troops’ withdrawal.
Ukraine and its Western allies previously accused Russian troops of war crimes. Moscow denies this.
It is not yet clear what happened to the victims, but some accounts have suggested many may have died from bombing and a lack of access to healthcare.
There are also signs that some of the graves could belong to Ukrainian soldiers.
Mr Tymoshko said exhumations would begin on Friday to get a clearer picture of the number of victims in the city, which was under Russian occupation for more than five months.
And in his address late on Thursday, President Zelensky said more “clear, verified information” would be presented on Friday to Ukrainian and foreign journalists.
“We want the world to know what is really happening and what the Russian occupation has led to. Bucha, Mariupol, now, unfortunately, Izyum,” he said.
The Ukrainian leader was referring to alleged mass graves found this spring in Bucha, near the capital Kyiv, and also near Mariupol – the key south-eastern Ukrainian port now occupied by Russian troops.
Andriy Yermak, the head of President Zelensky’s office, tweeted a photo of the alleged mass grave, also saying that more information was expected on Friday.
Much of Izyum lies in ruins, with one local politician telling reporters that up to 80% of the town’s infrastructure has been destroyed, and bodies are still being discovered in the rubble.
A mass burial was found in Izyum, Kharkiv region. Necessary procedures have already begun. All bodies will be exhumed and sent for forensic examination. Expect more information tomorrow.
Russia is a murderer country. A state sponsor of terrorism. pic.twitter.com/7pKTrYvlUF
— Andriy Yermak (@AndriyYermak) September 15, 2022
Izyum and a number of other cities in the Kharkiv region were liberated earlier this month during a swift Ukrainian counter-offensive that appeared to have surprised Russian troops and left them unprepared to defend their positions.
Ukraine says it has identified more than 21,000 possible war crimes – including killing civilians and rape – committed by Russian troops since President Vladimir Putin ordered a full-scale invasion of the country on 24 February.
Investigators and journalists found what appeared to be evidence of the deliberate killing of civilians in Bucha and other nearby areas.
Ukrainian forces said they found mass graves and evidence that civilians had been killed after their feet and hands were bound.
The International Criminal Court has already sent a team of investigators and forensics experts to Ukraine to investigate this.
US President Joe Biden and former UK Prime Minister Boris Johnson have both accused Russia of carrying out war crimes in Ukraine.
The Russian government has repeatedly denied targeting civilians, accusing Ukraine and the West of fabricating evidence.
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