Residents of the Ukrainian city of Izyum searched for dead relatives in a nearby wooded mass grave as emergency workers continued to exhume what they said were hundreds of bodies found.
The causes of death for those at the grave site, discovered last week after Russian forces were driven from the Kharkiv region, have not yet been established, although residents say some died in an air strike.
Ukrainian authorities have said at least one of the bodies had tied hands and rope marks on the neck.
Prosecutors in the Kharkiv region said, in an online statement, that they had found a basement where Russian forces allegedly tortured prisoners in the village of Kozacha Lopan, near the border with Russia, Associated Press reports.
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They released images which showed a Russian military TA-57 telephone with additional wires and alligator clips attached to it.
Ukrainian officials have accused Russian forces of using the Soviet-era radio telephones as a power source to electrocute prisoners during interrogation.
Duvets and sleeping bag are seen in a basement which, according to Ukrainian authorities, was used as a torture cell during the Russian occupation, in the retaken village of Kozacha Lopan, Ukraine.
Ukrainian president Volodymyr Zelenskiy said investigators had discovered new evidence of torture used against the people buried in Izyum, one of dozens of towns retaken in the northeastern Kharkiv after a lightning advance earlier this month.
“More than 10 torture chambers have already been found in the liberated areas of Kharkiv region, in various cities and towns,” Mr Zelenskiy said in a video address late on Saturday.
“Torture was a widespread practice in the occupied territories. That’s what the Nazis did — this is what (the Russians) do,” he added. “They will answer in the same way — both on the battlefield and in courtrooms.”
Clutching a neatly written list of names and numbers, resident Volodymyr Kolesnyk stepped between graves looking for relatives he said were killed in an air strike on an apartment building shortly before Izyum fell to Russian forces in April.
Mr Kolesnyk said he knew his relatives had been taken to the burial site and were in some of the graves marked with numbers.
He paused before a cross marked with the number 199 and after checking the list given to him by a local funeral company that dug the graves, carefully hung a small sign on it bearing the name of Yurii Yakovenko, his cousin.
Cross number 164, he said, was his cousin’s wife. And 174, his cousin’s mother, Kolesnyk’s aunt.
Moscow has not commented on the discovery of the graves. It regularly denies committing atrocities in the war or targeting civilians.
The head of the pro-Russian administration that abandoned the area earlier this month accused Ukrainians of staging the atrocities at the city of Izyum. “I have not heard anything about burials,” Vitaly Ganchev told Rossiya-24 state television.
Russian president Vladimir Putin has not responded to the accusations but he brushed off Ukraine’s swift counteroffensive, casting Russia’s invasion as a necessary step to prevent what he said was a western plot to break Russia apart.
“The Kyiv authorities announced that they have launched and are conducting an active counteroffensive operation,” Mr Putin said on Friday after a summit of the Shanghai Cooperation Organisation in the Uzbek city of Samarkand. “Well, let’s see how it develops, how it ends up,” he said with a grin.
Mr Putin has warned Moscow would respond more forcefully if its troops were put under further pressure, raising concerns he could at some point use unconventional means like small nuclear or chemical weapons.
US president Joe Biden, what he would say to Mr Putin if he was considering using such weapons, replied: “Don’t. Don’t. Don’t. It would change the face of war unlike anything since World War Two.”
The British Ministry of Defence warned on Sunday that Russia is likely to increase its attacks on civilian targets as it suffers battlefield defeats.
“In the last seven days, Russia has increased its targeting of civilian infrastructure even where it probably perceives no immediate military effect,” the ministry said in an online briefing.
“As it faces setbacks on the front lines, Russia has likely extended the locations it is prepared to strike in an attempt to directly undermine the morale of the Ukrainian people and government.”
Some military analysts have warned Russian might also stage a nuclear incident at the Russian-held Zaporizhzhia nuclear power plant, Europe's largest.
Russia and Ukraine have blamed each other for shelling around the plant that has damaged buildings and disrupted power lines needed to keep it cooled and safe.
One of the plant's four main power lines has been repaired and is once again supplying the plant with electricity from the Ukrainian grid, the United Nations nuclear watchdog said on Saturday.
Ukraine has also launched a major offensive to recapture territory in the south, where it hopes 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. — Agencies