Ukrainian President Volodymyr Zelensky on Wednesday denounced Russian "beasts" after a video appeared to show the beheading of a Ukrainian prisoner, prompting international outrage.
The emergence of the footage, which AFP was unable to independently verify, sparked fury in Ukraine, with Kyiv saying Russia is "worse than ISIS" — a reference to the Islamic State group.
The European Union pledged to hold war criminals to account, while a UN mission in Ukraine said it was "appalled".
The UN Human Rights Monitoring Mission in Ukraine also pointed to another video that is spreading online, saying it "shows mutilated bodies of apparent Ukrainian POWs."
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Moscow and Kyiv have repeatedly accused each other of killing prisoners of war since Russian President Vladimir Putin invaded Ukraine more than a year ago.
Several videos alleging to show POW killings have made the rounds on the internet over the past year.
Following the emergence of a new video, Mr. Zelensky said in an address posted to social media that "the world must see" what he called an "execution of a Ukrainian captive".
"There is something that no one in the world can ignore: how easily these beasts kill," a grim-looking Ukrainian leader said.
"We won't forget anything, nor will we forgive murderers," he added.
The video, which lasts around one minute 40 seconds, shows a masked man in camouflage decapitating another man in uniform.
After the screams stop, other voices are heard encouraging the attacker in Russian.
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After the man is decapitated, a voice is heard saying that the head should be sent "to the commander".
The victim's vest bearing the Ukrainian trident and a skull can also be seen in the footage.
"The UN Human Rights Monitoring Mission in Ukraine is appalled by particularly gruesome videos posted on social media," it said in a statement.
"One of the videos shows a brutal execution of a man who appears to be a Ukrainian prisoner of war, while the other one shows mutilated bodies of apparent Ukrainian POWs."
The EU said that, if confirmed, the footage was "yet another brutal reminder about the inhumane nature of the Russian aggression."
"The EU reiterates its firm commitment to holding to account all perpetrators and accomplices of war crimes committed in connection with Russia's war," EU spokeswoman Nabila Massrali said, adding that Brussels did not have information on the veracity of the video.
Mr. Putin's spokesman called the images "horrible" but said the authenticity of the video has to be confirmed.
"First of all, the authenticity of this horrible footage needs to be verified," Kremlin spokesman Dmitry Peskov told reporters.
"We live in a world of fakes," he added.
Top Ukrainian officials renewed calls to remove Russia from the UN Security Council where it is a permanent member.
"It's absurd that Russia, which is worse than ISIS, is presiding over the UNSC," wrote Foreign Minister Dmytro Kuleba.
Russia holds the rotating Security Council presidency for the month of April.
Mykhailo Podolyak, a political adviser to Mr. Zelensky, warned against "any attempts to find a compromise with the terrorists", adding that Russia must be isolated from "the rest of the world."