(CNN) Videos alleged to show Russian soldiers castrating and killing a bound and gagged Ukrainian soldier are circulating on social media, drawing an accusation of potential war crimes from the United Nations and prompting a Ukrainian investigation into possible torture.
The three videos appear to show a soldier wearing what look like yellow and blue patches -- the colors of the Ukrainian flag -- on the ground, with his hands tied behind his back, first being gagged, then mutilated, then shot.
CNN cannot independently verify the authenticity of the videos which circulated widely on social media after first appearing on a pro-Russian Telegram channel. CNN also has not independently verified the nationality of the perpetrators or the victim.
The UN Human Rights Monitoring Mission in Ukraine said it is appalled by the videos. In a statement issued late Friday, the Monitoring Mission said the "torture and summary executions of prisoners of war and persons hors de combat are war crimes."
The Prosecutor General's office in Ukraine said it has launched a criminal investigation into the matter, "based on the results of the video processing, which shows how persons in the uniform of servicemen of the Armed Forces of the Russian Federation are torturing a prisoner in the uniform of a serviceman of the Armed Forces of Ukraine. Measures are being taken to establish all the circumstances of the crime."
The video that appears to come first shows the victim being gagged surrounded by at least four men, one of whom is wearing what looks like a Russian uniform with the "Z" insignia associated with Russia's invasion of Ukraine.
A second video shows the same man writhing on the ground as a man wearing blue surgical gloves and what appears to be a Russian uniform cuts off the victim's underwear with a box cutter. The man uses the same box cutter to castrate him, then holds the mutilated genitals up for the camera. In the video, the perpetrators can be heard saying "hold him, hold him" in Russian.
The third video shows the bound and mutilated man being shot in the head and dragged away.
CNN has contacted the Russian Ministry of Defense for comment about the videos.
The UN Monitoring Mission said on Facebook it "is appalled by the latest videos, apparently showing the beating, castration and shooting of a captured soldier from the Ukrainian Armed Forces by a man, who appears to be a member of the Russian armed forces or affiliated armed groups."
"In one of the videos the tortured soldier appeared to be shot in the head and his body dragged into a ditch. If confirmed, these actions would constitute war crimes."
The UN Monitoring Mission said: "Unfortunately, this is only the latest in a series of videos that have documented torture and extrajudicial killings of prisoners of war and persons hors de combat, committed by belligerent parties."
"HRMMU has publicly reported on videos showing Ukrainian soldiers shooting the legs of three captured Russian soldiers and in a separate case shooting and killing an injured Russian soldier," the UN Monitoring Mission wrote.
It added: "All parties must refrain from torture, killings and threats of giving no quarter to captured combatants. They must promptly and effectively investigate all allegations of torture and unlawful killings."
High Representative of the European Union for Foreign Affairs and Security Policy Josep Borrell on Friday strongly condemned "Russia's ongoing illegitimate and unjustified war of aggression against Ukraine and its people brings further horrific atrocities day by day."
"Evidence in form of appalling video footage has been widely shared on pro-Kremlin social networks today, in which Russian soldiers commit a heinous atrocity against a Ukrainian prisoner of war," Borrell said in a statement.
Amnesty International has demanded an investigation into the videos.
"This horrific assault is yet another apparent example of complete disregard for human life and dignity in Ukraine committed by Russian forces. All those suspected of criminal responsibility must be investigated," said Marie Struthers, Amnesty International's Director for Eastern Europe and Central Asia.
"Since the start of Russia's war on Ukraine, Amnesty International has documented crimes under international law, like summary killings of captives by Russia-backed separatist forces in eastern Ukraine and extrajudicial executions of Ukrainian civilians by Russian forces," she said.