Ukraine says hundreds of graves have been found outside Izyum, days after the town was re-taken from Russia.
Wooden crosses, most of them marked with numbers, were discovered in a forest outside the town by advancing Ukrainian forces, BBC reported.
Authorities said they would start exhuming some of the graves.
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.
There are also signs that some of the graves could belong to Ukrainian soldiers, the report said.
Regional police head Volodymyr Tymoshko told the BBC more than 400 bodies were thought to have been buried at the site.
Izyum, invaded in the early days of the war, was used 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 in the area.
“We want the world 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 accountable for that.”
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.