About 440 bodies have been discovered in Izyum, Kharkiv region, Ukraine.
It is not yet clear what happened to the victims, but early accounts suggest some may have died from shelling and a lack of access to healthcare.
Speaking on Friday, the head of Ukraine's national police service said most of the bodies belonged to civilians.
Ihor Klymenko told a news conference that although soldiers were also believed to be buried there, there was so far no confirmation.
Earlier, Ukrainian authorities told the BBC more than 400 bodies were thought to be buried at the site.
Mykhailo Podolyak, a senior adviser to Ukrainian President Volodymyr Zelensky, that "many places of mass burials" had been discovered in some liberated areas.
"We saw many places where people were tortured," Mr Podolyak said. "We saw wildly frightened people who were kept without light, without food, without water, and without the right to justice. Because there was no authority there, there were only people with weapons."
President Zelenskyy likened the suffering in Izyum to the atrocities committed by Russian forces in the town of Bucha, just outside Kyiv, and the city of Mariupol, in the south.
He had ventured to the war-shattered city on Wednesday despite it standing close to Russia's frontline. Ukrainian soldiers only recaptured the area days earlier.
"Russia leaves death everywhere," Mr Zelenskyy said. "It must be held accountable for that. The world must hold Russia to real account for this war."
After all that has already been discovered one has to wonder what is left to yet be found.
