A videograb shows an alleged Russian solider sitting on top of an alleged Ukrainian soldier.

Ukraine launched an investigation on Wednesday into a gruesome video circulating on social media that purportedly shows the beheading of a Ukrainian soldier.

The video spread quickly online and sparked outrage from Ukrainian officials. The Kremlin called the footage "horrible” but said it needed to be verified.

Meanwhile, a Russian defence official claimed that fighters from Russia’s paramilitary Wagner group have seized three districts of Bakhmut, the embattled city that for months has been the focus of Moscow’s grinding campaign in the east.

The video circulating online appears to show a man in green fatigues wearing a yellow armband, typically donned by Ukrainian fighters.

He is heard screeching before another man in camouflage uses a knife to decapitate him.

A third man holds up a flak jacket apparently belonging to the man being beheaded. All three men speak in Russian.

Ukrainian President Volodymyr Zelensky on Wednesday denounced Russian "beasts" after a video appeared to show the beheading of a Ukrainian prisoner, prompting international outrage.

A UN mission in Ukraine said on Wednesday it was "appalled" by "gruesome" videos including footage showing an alleged decapitation of a Ukrainian prisoner of war.

"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."

The EU pledged on Wednesday to hold war criminals to account as it denounced footage spread online allegedly showing the beheading of a Ukrainian prisoner of war by Russian forces.

"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 added.

Ukraine compared Russia on Wednesday to Daesh and called on the International Criminal Court to investigate after a video emerged online showing apparent Russian soldiers filming themselves beheading a Ukrainian captive with a knife.

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 Daesh, is presiding over the UNSC," he said, referring to the UN Security Council, where Russia took up the rotating presidency this month.