Mikhailo Podolyak, a journalist, negotiator and adviser to the Ukrainian president, was among local officials who on Friday confirmed the authenticity of the video, vowing to "identify and get to each" of the invading "executioners."
He said "the world has to realise" that Russia is "a country of cannibals who enjoy torture and murder."
It is not clear when or where the video was filmed.
News.com.au has not independently verified the footage, however, Aric Toler, from investigative website Bellingcat which successfully identified the Salisbury poisoners, told The Times that the video is likely genuine.
"The same soldier appeared on a Russian TV clip (with same hat and bracelet) and there was no evidence the video had been manipulated," Toler said.
'Unmitigated evil'
Paul Massaro, a senior policy adviser for the Commission on Security and Cooperation in Europe, said the "sickening evil committed by the Russian invaders" should be a message for the world to "help Ukraine end it."
"The barbarity and depravity of the Russian invaders is revolting. Castration, murder, rape, killing children, levelling cities. Unmitigated evil," he tweeted.
John Spencer, the US Army veteran who chairs the Washington, DC-based Urban Warfare Studies, also implored world leaders to act.
"I just watched the video showing Russian soldiers cutting off the genitalia of Ukrainian POWs. How do you unsee that? WTF are we waiting on to halt this evil?" he wrote.
There are varying reports about who the Russian soldier is, as well as where and when the alleged incident took place.
The Twitter account Ukraine Alert, which tweets updates about the war, identified the soldier as "probably" a member of the Wagner group of mercenaries fighting alongside Moscow's forces in Ukraine, who is from the republic of Kalmykia.
Ukraine Alert pointed out that other sources had said the video was of mercenaries from the Chechen Akhmat battalion.
Several other outlets reported that the Russian appeared to be the same soldier who appeared in a June broadcast on Russian state-backed media at the Azot chemical plant in the city of Severodonetsk, Luhansk.
Retired Australian general Mick Ryan tweeted that while he did not want to amplify a "hideous and repulsive act against a defenceless Ukrainian prisoner of war … from a purely human level, I am sickened that one human will do this to another.
"Second, as a soldier, it breaks my heart to see a fellow soldier – now a non-combatant – being treated in such a way. No soldier deserves such disgusting treatment," he added.
'Russia has to pay'
The video was also shared on Twitter by a Ukrainian MP Inna Sovsun who said she was banned from the site after posting the clip on the social media platform.
She had written in her original tweet: "Russian soldier from Chechen battalion Ahmat cut off the genitalia of Ukrainian POW (prisoner of war).
"This is what Nazis are doing to Ukrainians.
"Russia has to pay for it!
"Give Ukraine the weapons we need to stop this nightmare once and for all. The world can't pretend like this isn't happening!"
Ms Sovsun later regained access to her account and wrote in a follow-up tweet on Friday: "Twitter banned my profile today. Because I posted a video where a Russian soldier castrates a Ukrainian POW.
"@Twitter decided it was too cruel. But this is what happens. And deleting the video won't change that.
"People should know what #Russia is doing!"
'Brutal war crimes'
Ukraine's Ministry of Foreign Affairs condemned the "brutal war crimes", calling on the International Criminal Court to investigate.
"Ukraine strongly condemns the brutal war crimes committed by the servicemen of the Russian Federation against Ukrainian prisoners of war, particularly horrible cases of torture, physical abuse, inhuman treatment, wilfully causing great suffering or serious injury to body or health and wilful killing of Ukrainian prisoners of war," the department said in a statement.
"Minister of Foreign Affairs Dmytro Kuleba calls on the international community to condemn the brutal violation of international law by the Russian Federation and to immediately recognise Russia as a terrorist state.
"He emphasised that any delay in this decision will only encourage Russia to commit further crimes and inhumane acts."
The International Criminal Court has described Ukraine as a "crime scene", sending its largest ever team to investigate.
Since the invasion of Ukraine began in February, Russia has been accused of committing thousands of war crimes. The allegations include the direct targeting of civilians, torture, rape and forced deportations.
In a report released earlier this month, the Organisation for Security and Co-operation in Europe detailed evidence that Russian forces in Ukraine have, indeed, committed many war crimes - which Russia denies.
