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... we won't forget anything, nor will we forgive murderers" Zelensky said in a message on social media, vowing to bring justice to his war-torn nation. 

The Kremlin said the authenticity of the "horrible" images had to be verified.

"First of all, the authenticity of this horrible footage needs to be verified," Kremlin spokesman Dmitry Peskov told reporters, adding that "we live in a world of fakes."

The United Nations 'appalled'

A UN mission in Ukraine said it was "appalled" by the "gruesome" videos including footage showing the alleged decapitation.

"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."

EU vows to hold war criminals to account

The EU pledged to hold war criminals to account after the footage spread online. 

"We don't have more information on the veracity of the video. Having said that, if confirmed, this is yet another brutal reminder about the inhumane nature of the Russian aggression," EU spokeswoman Nabila Massrali said. 

"The EU reiterates its firm commitment to holding to account all perpetrators and accomplices of war crimes committed in connection with Russia's war," she said.