President Volodymyr Zelensky said on Friday many Ukrainians were buried at various sites in the newly-recaptured northeast including whole families and people who were tortured, likening the aftermath to Russia's withdrawal from near Kyiv months ago.
In an interview at his presidential office, he told Reuters an investigation was underway with international assistance and that there was evidence of Russian war crimes in those areas.
"As of today, there are 450 dead people, buried. But there are others, separate burials of many people. Tortured people. Entire families in certain territories," he said.
Asked if there was evidence of war crimes, he said: "All this is there. Investigative commissions with international partners, joint investigative commissions," he said.
"Our prosecutors are also working with international ones. There is some evidence, and assessments are being conducted, Ukrainian and international, and this is very important for us, for the world to recognise this."
The governor of Kharkiv region, Oleh Synhubov, told reporters on Friday at one of the burial sites in the city of Izium that some bodies exhumed there had been found with their hands tied behind their backs.
Kyiv officials said they had counted 450 graves at the mass burial site and found 10 alleged "torture centres" after the Kharkiv region was recaptured from Russian invaders.
In the forest outside Izyum, AFP journalists saw graves topped with makeshift crosses and marked with numbers, with one inscription reading: "Ukrainian army, 17 people. Izyum morgue."
Kyiv's forces recovered a swathe of territory in recent days in a lightning counter-offensive in the east, liberating several towns from Russian forces but also uncovering what they say is a grim legacy of occupation.
Police chief Igor Klymenko said torture rooms were found in the town of Balakliya and elsewhere in Karkhiv, while presidential aide Mykhaylo Podolyak said the Izyum mass grave site alone held at least 450 bodies.
"In the occupied territories, rampant terror, violence, torture and mass murders have been reigning for months," Podolyak said.
On the main road from Izyum to Kharkiv, a small dirt road leads into a pine forest. On the right-hand side of the lane, about 100 metres (330 feet) into the trees, two men in white overalls were digging the sandy soil.
Soon they reached the first body, exhumed it and placed it in a white plastic body bag. As more bodies appeared, the strong smell of decay spread among the trees and rough wooden crosses.
Where identification had been possible, names were attached to the crosses along with dates between early March, when Izyum was still held by Ukraine, and early September, a period of Russian control.
On some of the graves, small offerings of flowers were placed in homage to the deceased.
According to Oleg Kotenko, the government official in charge of the search for missing persons nationwide, a family with a young child was buried there.
"They were killed. There are witnesses from the same building. They saw what happened and buried these people here," he said.
According to Kotenko: "The graves without names are for those found dead in the street."
The United Nations in Geneva said it hopes to send a team to determine the circumstances of the deaths in the forest graves.
Russia has been accused of carrying out attacks on civilians that could amount to war crimes, notably in suburban towns outside the capital of Kyiv after fighting in March.
Dozens of civilians bearing signs of extrajudicial killings were found in places like Bucha, outside Kyiv, after they were recaptured by Ukraine's forces earlier this year.
