Ukrainian officials say they have found hundreds of bodies, some with their hands tied behind their backs, buried in territory recaptured from Russian forces in what President Volodymyr Zelenskiy calls proof of war crimes by the invaders.
Zelenskiy told Reuters in an interview many corpses were also interred in other sites in the northeast and appealed for foreign powers to step up weapons supplies, saying the outcome of the war hinged on their swift delivery.
"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," Zelenskiy said in his presidential office on Friday.
The head of the pro-Russian administration which abandoned the area last week accused Ukrainians of staging the atrocities at the city of Izium.
"I have not heard anything about burials," Vitaly Ganchev told Rossiya-24 state television.
Outside the city on Friday, officials wearing masks and protective suits were digging out bodies at the site in a forest where about 200 makeshift wooden crosses were scattered among trees.
"We are at the site of the mass burial of people, civilians who were buried here, and now according to our information they all have the signs of violent death," Kharkiv Governor Oleh Synehubov said at the site.
"There are bodies with hands tied behind (their backs). Each fact will be investigated and will be properly and legally evaluated," Synehubov said.
Officials laid the exhumed bodies out in white body bags.
Some were wrapped in rags.
One was in military fatigues.
In a separate video address on Izium, Zelenskiy said authorities had found a mass grave containing the bodies of 17 soldiers, some of which bore obvious signs of torture.
US Secretary of State Antony Blinken said he had heard the reports.
"This is part, horrifically, of... an ongoing story. Whenever we see the Russian tide recede from the parts of Ukraine that it has occupied, we see what's left in its wake."
White House national security spokesman John Kirby called the accounts "horrifying".
Russian President Vladimir Putin did not immediately respond to the accusations but brushed off Ukraine's lightning counter-offensive and warned that Russia would respond more forcefully if its troops were put under further pressure.
Speaking after a summit of the Shanghai Cooperation Organisation in the Uzbek city of Samarkand, Putin cast the invasion as a necessary step to prevent what he said was a foreign plot to break Russia apart.
"The Kiev authorities announced that they have launched and are conducting an active counter-offensive operation. Well, let's see how it develops, how it ends up," Putin said with a grin.
Russia regularly denies targeting civilians during what it calls its "special military operation" in Ukraine and has said in the past that accusations of human rights abuses are a smear campaign.
In Kupiansk, a northeastern railway junction city whose partial capture by Ukraine's forces on Saturday cut Russia's supply lines and led to the swift collapse at the front, small units of Ukrainian troops were securing a nearly deserted ghost town.
A formerly Russian-occupied police station had been hastily abandoned.
Russian flags and a portrait of Putin lay on the floor amid broken glass.
Records had been torched.
After a week of rapid gains in the northeast, Ukrainian officials have sought to dampen expectations that they could continue to advance at that pace.
They say Russian troops that fled the Kharkiv region are now digging in and planning to defend territory in neighbouring Luhansk and Donetsk provinces.
Ukraine has also launched a major offensive to recapture territory in the south, where it aims to trap thousands of Russian troops cut off from supplies on the west bank of the Dnipro river, and retake Kherson, the only large Ukrainian city Russia has captured intact since the start of the war.
Russia's state-run RIA news agency released video showing smoke billowing from Kherson's Russian-occupied administration building after apparent Ukrainian rocket attacks that it said had killed three people and wounded 13.