Last week, 450 graves were discovered in forests outside Izium after the Ukrainian city was recaptured from the Russians. Ukrainian officials claim that 99 percent of the exhumed bodies show signs of violent death. The Kremlin has denied the allegations as "lies", while the EU presidency has called for the creation of an international war crimes tribunal.
In an interview with RFI and France 24 on Sunday, UN Secretary-General Antonio Guterres said he hopes the International Criminal Court will be able to investigate the alleged massacre in the eastern Ukrainian city.
The authorities in Kyiv have said they will use this week's annual general assembly of the United Nations, due to open in New York tomorrow, to call for the establishment of an international court to investigate alleged war crimes by Russian forces in Ukraine.
The EU presidency at the weekend called for the establishment of an international tribunal for war crimes.
"In the 21st century, such attacks against the civilian population are unthinkable and abhorrent," said Jan Lipavsky, foreign minister of the Czech Republic which holds the European Union's rotating presidency.
"We must not overlook it. We stand for the punishment of all war criminals," he added in a message on Twitter.
"I call for the speedy establishment of a special international tribunal that will prosecute the crime of aggression."
Ukraine's national police chief has said more than 10 "torture chambers" had been found in the formerly Russian-controlled parts of the northeastern Kharkiv region.
President Volodymyr Zelensky last week announced the discovery of what he called a "mass grave" in Izium after it was recaptured from the Russians.
At one burial site, more than 440 graves dating between March and September 2022 had been discovered.
Confusion over nature of graves
Investigators have exhumed the bodies of at least 17 Ukrainian soldiers from one. A cross over the grave bore the inscription: "Ukrainian army, 17 people. Izium morgue."
The authorities say there are more than 440 tombs.
Some of the graves are marked by crosses made from varnished wood carrying names and dates.
Investigators say about 100 bodies have been exhumed.
Ukrainian officials suspect that some of the dead were tortured by Russian forces during their occupation of the northeast Kharkiv region.
At least two of the bodies recovered were found with their hands tied.
One of the two had his hands tied, a broken jaw and two stab wounds in the back, a member of the Kharkiv prosecutor's office said.
The remains have been identified as those of a pro-Ukrainian volunteer fighter, the official added.
Civilian and military victims
Civilians who died during fighting in March for control of the city have also been exhumed.
Kharkiv prosecutor Yevgen Sokolov, who is leading the investigation, said he did not have an exact number for those thought to have suffered violent deaths.
Of the bodies so far recovered, he said "most have wounds from shelling and explosions".
Others had suffered "injuries from sharp objects and showed signs of violent death", he said.
Sokolov confirmed that one combatant had had "his hands tied behind his back" and another was found with "rope around the neck and broken limbs".
He also said a body had been found "with multiple stab wounds".
"At this point we don't have bodies with bullets in their skulls but there is still a lot of work to be done," he said.
If the weather remained mild, he estimated it would take another week to finish exhuming the bodies.
