Ukrainian officials have opened a probe into allegations that Russian prisoners of war were tortured.
A sickening video shared widely on social media appears to show a senior commander admitting to "war crimes".
At least three Russian troops appear to have been shot in the legs after being detained, and some have white bags on their heads.
In the distressing video, one loses consciousness or dies, say reports.
The Ukrainians appear to be quizzing the men in order to identify Russian officers.
The “torture” allegedly took place in the village of Olkhovka (Vilkhivka) on the eastern outskirts of Kharkiv, on Friday.
The footage has caused outrage in Russia, but Oleksiy Arestovych, a Ukrainian presidential adviser to Volodymyr Zelensky, has also acknowledged that this “massacre” amounted to a “war crime”, and promised a full investigation.
"I can say we have an extremely strict attitude towards such cases,” he said, referring to the footage.
“There will be an investigation. I have repeatedly said officially that we do not torture prisoners.”
Ukraine ’s army was “European” and while there would “always be excesses”, a
“massacre of prisoners of war is against the laws of military honour and is a war crime”.
He vowed: “There will be a very serious investigation, and those guilty, if found, will be punished.”
He said that “every member of the defence forces will be told that this is absolutely unacceptable behaviour, it is a war crime”.
He added: “More precisely, what is happening there has the characteristics of a war crime, a crime that can only be qualified by a court, which has no statute of limitations, no amnesty in Ukrainian or international law.”
Russia has separately opened a criminal investigation into the “extreme cruelty” of Ukrainian “nationalists” in their treatment of Russian military captives.
“Footage appeared on the Internet in which Ukrainian nationalists treat prisoners extremely cruelly,” said a statement from the Russian Investigative Committee.
“A video circulating on the net shows how captured soldiers were shot in the legs without medical assistance.
“According to some reports, illegal actions took place at a base of Ukrainian nationalists in the Kharkiv region.”
Ukraine’s military commander-in-chief Valeriy Zaluzhniy did not specifically deny the authenticity of these videos but claimed Russia “produces and shares videos with inhuman treatment of alleged ‘Russian prisoners’ by ‘Ukrainian soldiers’ in order to discredit Ukrainian defence forces.
He stressed that “service members of the Armed Forces of Ukraine and other legitimate military formations strictly adhere to the norms of International humanitarian law.
“I urge you to take into account the realities of informational and psychological warfare and trust only official sources."
