Ukrainian President Volodymyr Zelensky has denounced Russian "beasts" after a video appeared to show the beheading of a Ukrainian prisoner, prompting international outrage.
The emergence of the disturbing footage, which AFP was unable to independently verify, sparked fury in Ukraine, with Kyiv saying Russia is "worse than ISIS", a reference to the Islamic State group.
The European Union pledged to hold war criminals to account, while a UN mission in Ukraine said it was "appalled".
The UN Human Rights Monitoring Mission in Ukraine also pointed to another video that is spreading online, saying it "shows mutilated bodies of apparent Ukrainian POWs".
It was not clear when or where the video was filmed. Ukrainian authorities said they were trying to identify the victim.
Moscow and Kyiv have repeatedly accused each other of killing prisoners of war since Russian President Vladimir Putin invaded Ukraine more than a year ago.
A number of videos alleging to show POW killings have made the rounds on the internet over the past year.
Following the emergence of a new video, Mr Zelensky said in an address posted to social media that "the world must see" what he called an "execution of a Ukrainian captive".
"There is something that no one in the world can ignore: how easily these beasts kill," the grim-faced Ukrainian leader said.
"We won't forget anything, nor will we forgive murderers," he added.
Mr Zelensky urged world leaders to act and help Ukraine defeat Russia as Kyiv forces prepare a counter-offensive after a months-long stalemate on the battlefield.
"We need to act now!" the Ukrainian president said.
The video, which lasts around one minute 40 seconds, shows a masked man in camouflage decapitating another man in uniform.
After the screams stop, 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 and a skull can also be seen in the footage.
The SBU security service said it had opened a probe into the video.
"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."
'Inhumane aggression'
The EU said that, if confirmed, the footage was "yet another brutal reminder about the inhumane nature of the Russian aggression".
"The EU reiterates its firm commitment to holding to account all perpetrators and accomplices of war crimes committed in connection with Russia's war," EU spokeswoman Nabila Massrali said. The EU did not have information on the veracity of the video, she added.
Kremlin spokesman Dmitry Peskov told reporters: "First of all, the authenticity of this horrible footage needs to be verified. We live in a world of fakes."
Top Ukrainian officials renewed calls to remove Russia from the UN Security Council where it is a permanent member.
"It's absurd that Russia, which is worse than ISIS, is presiding over the UNSC," wrote Foreign Minister Dmytro Kuleba.
Russia holds the rotating Security Council presidency for the month of April.
Mykhailo Podolyak, a political adviser to Mr Zelensky, warned against "any attempts to find a compromise with the terrorists".
Russia had to be isolated from "the rest of the world", he said.
UK casts doubt on reports of special forces in Ukraine
Britain's Ministry of Defence (MoD) has cast doubt on reports that UK special forces have operated in Ukraine, after the allegations emerged from a leaked haul of highly sensitive US documents.
The Guardian and other media outlets said the leaked military and intelligence files suggest that dozens of British special forces personnel have been present in Ukraine this year.
The classified US material, which has not been verified, was leaked and then circulated on social media.
The British Embassy in Kyiv said in June 2021 that UK and US special forces were holding "joint training activity" with their Ukrainian counterparts.
But London has not disclosed their continued presence inside the country since Russia's invasion in February last year.
"The widely reported leak of alleged classified US information has demonstrated a serious level of inaccuracy," the MoD said in a statement posted on Twitter.
"Readers should be cautious about taking at face value allegations that have the potential to spread disinformation."
US Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin spoke with their Ukrainian counterparts yesterday as Washington seeks to reassure its allies following the leak.
Russia said it was unclear whether the leaked documents were real, but that it had already known that instructors from NATO countries were taking part in military actions in Ukraine.
The breach - which has sparked a criminal investigation by the Department of Justice - includes various classified information about Ukraine's battle against Russian forces, as well as secret assessments of US allies.
Many of the documents are no longer available on the sites where they first appeared, and the United States is reportedly working to have them removed.
