Ukrainian President Volodymyr Zelensky said on Thursday that a mass burial site has been found in a northeastern city that until a few days ago was held by Russian forces.

"A mass burial site was found in Izium, Kharkiv region. The necessary procedural actions have already begun there. There should be more information — clear, verified information — tomorrow," Zelensky said in his nightly address.

"We want the world to know what is really happening and what the Russian occupation has led to," the Ukrainian president continued. "Bucha, Mariupol, now, unfortunately, Izium... Russia leaves death everywhere. And it must be held accountable for that."

Moscow's forces withdrew from Izium and other parts of the Kharkiv region last week following a Ukrainian counteroffensive which prompted thousands of Russian troops to flee.

Police: More than 400 bodies found

Also on Thursday, Serhiy Bolvinov, the chief police investigator for Kharkiv region, told UK broadcaster Sky News that forensic investigations would be carried out on every corpse found in the mass grave.

"I can say it is one of the largest burial sites in a big town in liberated (areas)... 440 bodies were buried in one place," Bolvinov said.

"Some died because of artillery fire ... some died because of air strikes," he said.

On Wednesday, Zelensky made a rare trip out of Kyiv to watch the raising of the national flag at Izium's city hall.