Ukrainian President Volodymyr Zelenskyy 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," Zelenskyy 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

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)," Bolvinov said, adding that 440 bodies were buried in the one area.

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

Before beginning to exhume the bodies, investigators scanned the site with metal detectors to check for any hidden explosives.

Graves marked with numbers

Associated Press journalists visited the site and reported hundreds of graves with simple wooden crosses, most of them marked only with numbers. A larger grave bore a marker saying it contained the bodies of 17 Ukrainian soldiers.

Zelenskyy's chief of staff Andriy Yermak accused Russia of murder in a tweet about the burial site. He included a photo of the crudely made wooden crosses.

Russia has not yet commented on the discovery.

Zelenskyy himself made a rare trip out of Kyiv earlier on Wednesday to watch the raising of the national flag at Izium's city hall.

UN Human Rights Office to verify collective grave with monitoring trip to Izium 

The UN High Commissioner for Human Rights office said on Friday it planned to send a team to Izium to verify the claims by Ukraine that a mass grave had been found there after the city was retaken from the Russian army. 

"Our colleagues in Ukraine are following up on these allegations, and they are aiming at organizing a monitoring visit to Izyum to determine the circumstances of the death of these individuals," spokeswoman Elizabeth Throssell said. 

zc,jsi/wd (Reuters, AP, dpa, AFP)