Ukrainian President Volodymyr Zelenskyy spoke out Wednesday against a graphic video posted online that purports to show a Ukrainian prisoner of war being decapitated by a Russian fighter.

"This video, the execution of a Ukrainian captive, the world must see it," Zelenskyy said in a video statement Wednesday. "This is not an accident. This is not an episode. This was the case earlier. This was the case in Bucha. Thousands of times. Everyone must react. Every leader."

Ukrainian security services said the video was posted online Tuesday. In it, a victim appears to initially be alive as he is attacked with a knife.

CBS News was unable to independently verify the authenticity of the video or confirm where or when it was recorded.

Ukraine's security service said it had launched an investigation into the attack.

"We will find these monsters. If necessary, we will get them wherever they are: from under the ground or from another world. But they will definitely be punished for what they have done," the head of the security service, Vasyl Maliuk, said in a statement.

Kremlin spokesman Dmitry Peskov told reporters at a briefing Wednesday that, "The authenticity of this horrible footage needs to be verified. We live in a world of fakes."

A United Nations mission in Ukraine condemned the video, along with another one also posted online.

"The UN Human Rights Monitoring Mission in Ukraine is appalled by particularly gruesome videos posted on social media," the mission 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."

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