Ukrainian President Volodymyr Zelenskyy has called on world leaders to react after a video emerged online allegedly showing Russian soldiers beheading a Ukrainian captive with a knife.
Warning this story contains graphic depictions that may disturb some readers.
Reuters could not immediately verify the authenticity or provenance of the video on social media, which appeared to show a man in green fatigues wearing a yellow armband, typically donned by Ukrainian fighters.
He is heard screeching before another man in camouflage uses a knife to decapitate him.
A third man holds up a flak jacket apparently belonging to the man being beheaded.
All three men speak in Russian.
Ukraine compared the actions of Russian soldiers to those of Islamic State terrorists and called on the International Criminal Court to investigate.
"There is something that no one in the world can ignore: how easily these beasts kill," Ukrainian President Volodymyr Zelenskyy said in a video message.
"There will be legal responsibility for everything. The defeat of terror is necessary."
Ukrainian Foreign Minister Dmitro Kuleba tweeted: "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.
"Russian terrorists must be kicked out of Ukraine and the UN and be held accountable for their crimes."
Militants from Islamic State in Iraq and Syria were notorious for releasing videos of beheadings of captives when they controlled swathes of those countries from 2014-2017.
Ukraine's foreign ministry called on the International Criminal Court to "immediately investigate yet another atrocity of the Russian military".
Since Russia's forces invaded Ukraine more than a year ago, they have committed widespread abuses and alleged war crimes, according to the United Nations, and rights groups.
The International Criminal Court has also issued an arrest warrant for Russian President Vladimir Putin for war crimes.
The Kremlin described the video as "awful" but said its authenticity needed to be checked.
Moscow has previously denied that its troops have committed atrocities during the invasion of Ukraine.
In Moscow, Kremlin spokesperson Dmitry Peskov told reporters at a briefing: "First of all, in the world of fakes that we live in, we need to check the veracity of this footage".
"Then it could be a pretext to check whether or not this is true, whether it happened, and if it did, where and by whom."
Andrei Medvedev, a Russian state TV journalist and a member of the Moscow city legislature, speculated that the timing of the video's release was "fairly opportune" for the Ukrainian army, saying it could help "fire up personnel ideologically" ahead of a planned major counteroffensive.
The Kremlin denies it has committed war crimes or that it has targeted civilians.
Ukrainian troops have also been accused of abuses, and last year Kyiv said it would investigate video footage circulated online that Moscow alleged showed Ukrainian forces killing Russian troops who may have been trying to surrender.
Ukrainians urged to stop sharing video
Deputy Defence Minister Hanna Maliar told people online not to name the soldier publicly until his identity has been officially established by law enforcement.
She urged people to stop sharing the video online.
"Remember, the enemy wants to frighten us. Wants to make us weaker," she said.
Ukraine's domestic security agency said it had started an investigation into a suspected war crime over the video.
"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.
In Geneva, the UN Human Rights Monitoring Mission in Ukraine said it was appalled by what it called "particularly gruesome" videos posted on social media.
As well as the purported execution, another video shows mutilated bodies of apparent Ukrainian prisoners of war, it said.
"Regrettably, this is not an isolated incident," it said in a statement.
"The latest incidents must also be properly investigated and the perpetrators must be held accountable."
Wires/ABC
