Ukraine says many graves been found outside Izyum, days after the town was re-taken from Russia.
Wooden crosses, most of them marked with numbers, were discovered during a forest outside the town by advancing Ukrainian forces.
Authorities said they might start exhuming some of the graves on Friday.
It is not yet clear what happened to the victims, but early accounts suggest some may have died from shelling and a scarcity of access to healthcare.
There also are signs that some of the graves could belong to Ukrainian soldiers.
Regional police head Volodymyr Tymoshko told the BBC quite 400 bodies were thought to have been buried at the site.
Izyum, invaded within the early days of the war, was employed by Russia as a key military hub to supply its forces from the east.
In his nightly address, President Volodymyr Zelensky said the “necessary procedural actions” had begun within the area.
“We want the planet to know what is really happening and what the Russian occupation has led to. Bucha, Mariupol, now, unfortunately, Izyum… Russia leaves death everywhere,” he said. “And it must be held in charge of that.”
The Ukrainian leader was pertaining 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.
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Andriy Yermak, the top of President Zelensky’s office, tweeted a photograph of the alleged mass burial site, 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 within the rubble.
A mass burial was found in Izyum, Kharkiv region. Necessary procedures have already begun. All bodies are going to be exhumed and sent for forensic examination. Expect more information tomorrow.
Russia may be a murderer country. A state sponsor of terrorism. pic.twitter.com/7pKTrYvlUF
— Andriy Yermak (@AndriyYermak) September 15, 2022
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View original tweet on Twitter
Izyum and variety 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’s identified more than 21,000 possible war crimes – including killing civilians and rape – committed by Russian troops since President Putin ordered a full-scale invasion of the country on 24 February.
Investigators and journalists found what seemed 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 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 completing war crimes in Ukraine.
The Russian government has repeatedly denied targeting civilians, accusing Ukraine and therefore the West of fabricating evidence.