Ukrainian President Volodymyr Zelensky on Wednesday denounced Russian "beasts" after a video surfaced on social media purporting to show the decapitation of a Ukrainian prisoner of war.

"There is something that no one in the world can ignore: how easily these beasts kill," Zelensky said in a video address released on social media.

"We won't forget anything, nor will we forgive murderers," he added, vowing to bring justice to his war-torn nation.

"This video... The execution of a Ukrainian captive... The world must see it," Zelensky added in the video address.

AFP was unable to independently verify the footage.

The video, which lasts around one minute 40 seconds, shows a masked man in camouflage decapitating another man in uniform.

Other voices are heard encouraging the attacker in Russian.

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 can also be seen in the footage.

Moscow and Kyiv have repeatedly accused each other of killing prisoners of war since Russia invaded Ukraine more than a year ago, and several videos alleging to show POW killings have made the rounds on the internet.