Ukraine launched an investigation 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.

The Associated Press was not able to verify the authenticity of the video or the circumstances of where and when it was shot.

Meanwhile, a Russian defense 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.

Since Russia’s forces invaded Ukraine more than a year ago, they have committed widespread abuses and alleged war crimes, according to the United Nations, rights groups and reporting by The Associated Press. Ukraine has repeatedly accused Russia of targeting apartment buildings in its strikes, and images of hundreds of civilians lying dead in the streets and in mass graves in Bucha after Russian forces withdrew have horrified the world.

The International Criminal Court has issued an arrest warrant for Russian President Vladimir Putin for war crimes.

The Kremlin denies it has committed war crimes or that it has targeted civilians.

Ukrainian troops have also been accused of abuses, and last year Kyiv said it would investigate video footage circulated online that Moscow alleged showed Ukrainian forces killing Russian troops who may have been trying to surrender.

Ukrainian President Volodymyr Zelenksyy said the violence in the latest video would not be forgotten — and that Russian forces would be held responsible for such acts.

“Everyone must react, every leader. Do not expect that it will be forgotten, that time will pass,” he said in a video posted to his official Telegram channel.

In it, he used strong language to describe Russian soldiers, including calling them “beasts.”

Kremlin spokesman Dmitry Peskov said that it’s necessary to thoroughly check the “horrible” video — including verifying that it’s real.

“In the world of fakes we live in the authenticity of the footage must be checked,” he said in a conference call with reporters.

Ukraine’s state security service launched an investigation the video, according to a statement from Vasyl Maliuk, the head of the agency, known as the SBU.

Posters in some pro-Kremlin Russian Telegram channels, while not confirming the authenticity of the video, did not dispute it. Some sought to justify it by saying that Russian troops have become hardened by combat.

Andrei Medvedev, a Russian state TV journalist and a member of the Moscow city legislature, speculated that the timing of the video’s release was “fairly opportune” for the Ukrainian army, saying it could help “fire up personnel ideologically” ahead of a planned major counteroffensive.

Mykhailo Podolyak, an advisor to Zelenskyy, also linked the video’s release to the expected offensive, but said it was meant to “demoralize the public mood or at least change the psychological perception of the war right now.”

Ukraine Will Seek U.N. Investigation

Ukraine’s ombudsman on Wednesday said that he will request that the U.N. Human Rights Committee investigate the video. Dmytro Lubinets said he has also written letters to the U.N. Commissioner for Human Rights, the U.N. Monitoring Mission in Ukraine, the U.N. secretary-general and the International Committee of the Red Cross.

He wrote on Telegram that “a public execution of a captive is yet another indication of a breach of Geneva Convention norms, international humanitarian law, a breach of the fundamental right to life.”

The front lines of the war have been largely frozen for months, with much fighting focused around the city of Bakhmut.

In the latest of regular video briefings, Defense Ministry spokesman Igor Konashenkov said Wagner forces had made some progress there. Ukrainian officials did not immediately comment, but Zelenskyy has said before that his troops could pull out if they face a threat of being encircled by Russian forces.

Konashenkov did not specify which neighborhoods of Bakhmut are now under Russian control, or how much of the city remains in Ukrainian hands.

Elsewhere, at least four Ukrainian civilians were wounded as Russian forces shelled a Ukrainian-held town near the Zaporizhzhia nuclear power plant, regional Gov. Serhii Lysak said.

In a Telegram post, Lysak said that “people are being pulled out from under the rubble” after Russian shelling destroyed 13 houses and cars in Nikopol, across the Dnieper river from the nuclear plant.

Gov. Pavlo Kyrylenko alleged that Russian forces also attacked a town in the eastern Donetsk province with cluster munitions — which are banned by an international treaty — wounding one person. An AP and Frontline database called War Crimes Watch Ukraine has cataloged how Russia has used cluster bombs.