Human rights group Amnesty International said on Thursday at least four Ukrainian soldiers had been shot dead in what it described as "execution-style" killings by pro-Russian separatist groups earlier this year.
The London-based organization said the torture and killing of captured soldiers was a war crime and urged a full investigation into the deaths in eastern Ukraine.
Eduard Basurin, a spokesman for the rebel forces, denied all accusations of wrongdoing.
"They don't have the facts. Let them present the facts, photographs, video, then we can comment. Otherwise this simply destabilizes the situation," he told Reuters.
Reuters could not independently verify any of Amnesty's assertions.
More than 6,000 people have been killed since fighting broke out between the Moscow-backed separatists and Ukrainian government forces last year, but a fragile ceasefire has taken hold in recent weeks.
Amnesty said it had seen videos of at least four soldiers held captive by separatists in January and February. All four later died, including three who had been seized during a battle for the strategic rail junction of Debaltseve.
Their bodies were later photographed in a morgue.
"There are signs of bullet wounds to their heads and upper parts of their bodies, apparently the result of execution-style killings," Amnesty said.
Denis Krivosheev, the deputy director for Amnesty in Europe and Central Asia, said the evidence confirmed what the group had long suspected.
"The torture, ill-treatment and killing of captured, surrendered or wounded soldiers are war crimes," he said in a statement, calling for a prompt investigation and fair trials for those behind the deaths.
Amnesty also said it had seen video of a Ukrainian soldier named as Ihor Branovytsky, who was captured by separatists after taking part in the defense of the airport in the city of Donetsk in January.
Witnesses said he was shot dead at point-blank range by separatist commander Arseny Pavlov.
Pavlov, who is better known by his nom de guerre "Motorola", declined to comment when contacted by Reuters.
Earlier this week, the Kyiv Post, an English-language newspaper, posted a phone interview with a man it said was Pavlov. In the audio, the man says he had killed 15 captured Ukrainian government soldiers, but declines to comment specifically on the death of Branovytsky.
"I've shot 15 prisoners. I don't give a s**t. No comment. I kill whoever I want," the man said.
Reuters could not verify that the voice on the Kyiv Post audio was that of Pavlov.
In Kiev, a comrade of Branovytsky said he was there when his friend was killed. Yury Shkabura, 42, said he heard a doctor tell Pavlov that Branovytsky needed to be taken to hospital for medical attention.
"Five minutes later, Motorola came back and shot him twice. He stepped back and said: 'I have already cured him'," Shkabura told Reuters television.
A medical certificate shown by Branovytsky's mother stated he had died from two bullet wounds in his head.
At a briefing on Thursday, Ukrainian State Security official Vasil Vovk said Kiev authorities would try to put Pavlov on an international wanted list.
"This person will bear responsibility for crimes against humanity, torture, murder of Ukrainian citizens including servicemen of the Ukrainian armed forces," he said.
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