Ukraine says it has evidence of a mass grave in Izyum, the north-eastern city recently freed from Russian occupation.
Regional police head Volodymyr Tymoshenko 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.