The Ukrainian parliament's commissioner for human rights on Sunday denied Kyiv's forces had killed Russian prisoners of war, arguing that Ukraine's soldiers were defending themselves against Russians who feigned surrender.
Videos circulated on Russian social media this week purporting to show the bodies of Russian servicemen killed after surrendering to Ukrainian troops.
Ukrainian Ombudsman Dmytro Lubinets said "excerpts" of a video showed that Russians "using a staged capture... committed a war crime by opening fire on the Ukrainian Armed Forces."
This means the soldiers "cannot be considered prisoners of war," he argued.
"Those who want to use the protection of international law to kill must be punished," he added.
One video circulating on Russian social media shows soldiers apparently surrendering to several military personnel in camouflage and wearing yellow armbands.
The troops giving themselves up lie down on the ground in the debris-filled backyard of a house.
The video abruptly cuts off as shots are heard.
Another video filmed from above shows the bodies of around a dozen people surrounded by apparent blood stains.
AFP has not independently confirmed the videos.
The Russian Defense Ministry said on Friday the videos showed the "deliberate and methodical murder of more than 10 restrained Russian soldiers."
It called for an investigation into "war crimes."
Russian Foreign Ministry spokesperson Maria Zakharova condemned "the merciless shooting of unarmed Russian" prisoners and demanded that "international organizations condemn and thoroughly investigate this shocking crime."
A UN spokesperson told AFP on Friday it was "aware of the videos" and "looking into them."