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.
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.
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.