Ukraine claims hundreds of graves were found outside Izyum a few days after the city was recaptured from Russia.
Wooden crosses, most of them marked with numbers, were discovered in a forest outside the city by the advance of Ukrainian troops.
Authorities said they would start excavating some graves on Friday.
It is still unclear what happened to the victims, but early reports suggest that some have died as a result of the bombing and lack of access to health care.
There is also evidence that some of the graves may be of Ukrainian soldiers. Regional police chief Volodymyr Timochko said that more than 400 bodies are believed to have been buried at the site.
Izyum, occupied early in the war, was used by Russia as an important military center to supply its forces from the east.
In his overnight address, President Volodymyr Zelenskyy said “necessary procedural actions” had begun in the region.
“We want the world to know what is really happening and what the Russian occupation has led to. Bucha, Mariupol, now, unfortunately, Izyum … Russia is leaving death everywhere, “he said.” And it must be held responsible for that. ”
The Ukrainian leader referred to alleged mass graves found this spring in Bucha, near the capital Kyiv, and also near Mariupol, the main southeastern Ukrainian port now occupied by Russian forces.
Andriy Yermak, the head of President Zelensky’s office, tweeted a photo of the alleged mass cemetery and also said more information is expected on Friday.
Much of Izyum is in ruins, and a local politician told reporters that up to 80 percent of the city’s infrastructure has been destroyed and that bodies are still being discovered in the rubble