A mass burial site has been found in the Ukrainian city of Izyum, after it was liberated from Russian control.
It is thought to contain around 440 graves.
Ukraine's President Volodymyr Zelenskyy confirmed the discovery but said "clear, verified information" would be released later.
"We want the world to know what is really happening and what the Russian occupation has led to," Mr Zelenskyy said in a nightly video address to the nation on Thursday.
An image released by his Chief of Staff Andriy Yermak showed a number of wooden crosses jutting up from mounds in the earth, surrounded by trees.
He wrote: "A mass burial was found in Izyum, Kharkiv region. Necessary procedures have already begun.
"All bodies will be exhumed and sent for forensic examination. Expect more information tomorrow.
"Russia is a murderer country. A state sponsor of terrorism", he added.
Separately, Ukraine's Ministry of Defence confirmed the discovery, adding: "The current largest burial sights has 440 unmarked graves."
Mass graves are being discovered in Izyum after liberation from the russcists. The current largest burial sights has 440 unmarked graves.@ZelenskyyUa : "The necessary procedural actions have already begun there. More information - clear, verified - should be available tomorrow." pic.twitter.com/IipipvFJpb
— Defense of Ukraine (@DefenceU) September 15, 2022
Serhii Bolvinov is chief police investigator for the Kharkiv region.
He said: "I can say that there is one of the biggest burials in one liberated city, which contains more than 440 graves. Some 440 bodies were buried in one place."
Asked if he could say how the victims had died, he said: "We know that some were killed [shot dead], some died because of artillery fire, so-called mine explosion traumas.
"Some died because of airstrikes. Also we have information that a lot of bodies have not been not identified yet.
"So the reasons of death will be established during the investigations," he added.
Additional reporting: IRN