The clip appears to show a man in green fatigues wearing the yellow armband typically used by Ukrainian soldiers. He is heard screaming before another man apparently uses a knife to decapitate him.

Sky News has not been able to verify the authenticity of the video, which appears to have been taken by one of the apparent Russian soldiers.

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"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.

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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".