Zelenskyy labels Russian troops ‘beasts’ after a video appears to show a Ukrainian soldier being beheaded with a knife.
Al Jazeera was not able to independently verify the authenticity of the video, which spread quickly online.
The footage shows a man in uniform beheading a man who wears the yellow arm band used by Ukrainian soldiers.
The video is heavily blurred and contains a voice at the beginning, which suggests that the victim might have still been alive when the attack began.
The Kremlin described the video as “awful” and said its authenticity needed to be checked. Moscow has denied in the past that its troops have carried out atrocities during the conflict.
“There is something that no one in the world can ignore: how easily these beasts kill,” Ukrainian President Volodymyr Zelenskyy said.
“We are not going to forget anything. Neither are we going to forgive the murderers. There will be legal responsibility for everything. The defeat of terror is necessary,” he added.
Ukraine’s Foreign Minister Dmytro Kuleba described the video as “horrific”.
“It’s absurd that Russia, which is worse than ISIS, is presiding over the UNSC,” he wrote on Twitter, referring to the United Nations Security Council, where Russia took up the rotating presidency this month. “Russian terrorists must be kicked out of Ukraine and the UN and be held accountable for their crimes.”
ISIS (ISIL) was notorious for releasing videos of the beheadings of captives when they controlled swaths of Iraq and Syria from 2014-2017.
Ukraine’s domestic security agency (SBU) said it has launched an investigation into the suspected war crime.
“Yesterday, a video appeared on the Internet showing how the Russian occupiers are showing their beastly nature – cruelly torturing a Ukrainian prisoner and cutting off his head,” the SBU agency wrote on Telegram.
The European Union also pledged to hold war criminals to account.
“We don’t have more information on the veracity of the video. Having said that, if confirmed, this is yet another brutal reminder about the inhumane nature of the Russian aggression,” EU spokesperson Nabila Massrali said.
“The EU reiterates its firm commitment to holding to account all perpetrators and accomplices of war crimes committed in connection with Russia’s war,” she said.
The footage comes days after a separate video emerged on pro-Russian social media channels that appeared to show the bodies of two decapitated Ukrainian soldiers lying next to a destroyed military vehicle.
It is believed to have been filmed by members of the Wagner Group, a Russian mercenary organisation. The deceased Ukrainian soldiers appear to have had their hands cut off. Al Jazeera was unable to verify the authenticity of the video.