Stockholm, Sweden - A Russian ex-mercenary has identified former comrades as the perpetrators in the shocking video allegedly showing the beheading of a Ukrainian prisoner of war, according to civil rights activists.

They had sent the video footage to former Wagner mercenary commander Andrey Medvedev, who fled to Norway months ago and is currently imprisoned in Sweden, the founder of the Russian civil rights organization Gulagu.net, Vladimir Osechkin, said in an interview on Wednesday.

"He has listened to and watched it carefully several times and he clearly recognizes his former colleagues there, fighters from the mercenary Wagner troop," Osechkin said in the video, which was published on the YouTube channel of expatriate Russian opposition figure Mikhail Khodorkovsky.

Medvedev, who himself used to fight for Wagner, the notorious Russian mercenary group, had identified the men by "characteristic call signs and their way of speaking," the civil rights activist continued.

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Osechkin's organization is known primarily for its work combating violence in Russian prison camps. However, it also looked after Medvedev when he was arrested in Norway in January under that country's immigration law after fleeing.

Ukrainian President Volodymyr Zelensky has demanded that world leaders respond to the horrific footage, calling Russian forces "beasts" and "creatures."