KYIV, Ukraine (AFP) — About 450 graves were counted at one burial site near the eastern Ukraine city of Izyum which was recently recaptured by Kyiv's forces, a senior presidential aide said Friday.

"Four hundred and fifty graves… This is just one of the mass burial sites discovered near Izyum. In the occupied territories, rampant terror, violence, torture and mass murders have been reigning for months," Mykhaylo Podolyak said.

On Thursday, Ukrainian President Volodymyr Zelensky likened the graves to deaths seen in the cities of Bucha and Mariupol, which have become symbols of Russian atrocities.

"We want the world to know what the Russian occupation has caused," Zelensky said, without giving details on the number of bodies found or their cause of death.

Presidential chief of staff Andriy Yermak accused Russia of murder in a tweet about the mass grave, posting a photo of a forested area dotted with crudely made wooden crosses.

"All bodies will be exhumed and sent for forensic examination," he said. "Expect more information tomorrow."

A regional police official, Sergei Botvinov, told Sky News that some of the dead in the graves were shot and others were killed in shelling.

"Also we have information that a lot of bodies have not been identified yet. So the reasons of death will be established during the investigations," he added.