Volodymyr Zelenskyy condemned a disturbing video that appears to show a Russian soldier beheading a Ukrainian hostage.
In the film, a man wearing the yellow wristband normally worn by Ukrainian military appears to be wearing green fatigues.
Before another man appears to wield a knife to cut off his head, he can be heard screaming.
The footage, which appears to have been recorded by one of the presumed Russian soldiers, cannot be independently verified by Sky News as legitimate.
“There is something that no one in the world can ignore: how easily these beasts kill,” the Ukrainian president said, his face sombre.
“There will be legal responsibility for everything. The defeat of terror is necessary,” he added in a video message.
The Kremlin described the video as “awful” but said its authenticity needed to be checked.
Moscow has denied in the past that its troops carry out atrocities during the conflict.
Ukrainian foreign minister Dmitro Kuleba said on Twitter: “A horrific video of Russian troops decapitating a Ukrainian prisoner of war is circulating online.
“It’s absurd that Russia, which is worse than ISIS, is presiding over the UNSC,” he said, referring to the UN Security Council, where Russia took up the rotating presidency this month.
He added: “Russian terrorists must be kicked out of Ukraine and the UN and be held accountable for their crimes.”
Ukraine’s foreign ministry called on the International Criminal Court to “immediately investigate yet another atrocity of the Russian military”.
The UN says it is “appalled” by the footage.
“Regrettably this is not an isolated incident,” the UN Human Rights Monitoring Mission in Ukraine said in a statement.
It said it recently found “a number of serious violations of international humanitarian law”, including against prisoners of war.
“These latest violations must also be properly investigated and the perpetrators must be held accountable,” it said.
Ukraine’s ombudsman Dmytro Lubinets said today that he will call on the UN Human Rights Committee to investigate the video apparently showing the Ukrainian prisoner of war’s execution.