Images captured of metal cages being constructed at the philharmonic concert hall in occupied Mariupol appear to confirm alleged plans by Russian-backed separatists to stage 'show trials' for Ukrainian prisoners of war (POW).

The concert hall is reportedly set to be used to stage the controversial trials in the style of an 'open tribunal' for captured Ukrainian fighters, a process which the UN's rights office has said could itself amount to a war crime.

Footage of what appeared to be prison cells in the Mariupol Chamber Philharmonic Hall were released earlier this month by Mariupol City Council, which no longer controls the city.

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"We are very concerned about the manner in which this is being done," Ravina Shamdasani from the UN's Office of the High Commissioner for Human Rights (OHCHR) told a UN briefing on Tuesday.

"There are pictures in the media of cages being built in Mariupol's philharmonic hall, really massive cages and apparently the idea is to restrain the prisoners," she said. "This is not acceptable, this is humiliating."

If Moscow and its proxies were to purposely deprive a prisoner of war of the right to a fair trial, it would amount to a war crime by Russia, she added.

There had been speculation that pro-Russian authorities would try the Ukrainian POWs, including members of Ukraine's ultra-nationalist Azov Regiment, on Wednesday, which was both Ukraine's Independence Day and the six month anniversary of Moscow launching its military campaign against its neighbor. 

The head of the Russian-backed separatist administration in eastern Ukraine's Donetsk region said earlier this month that a trial of captured personnel from the Azov Regiment would take place by the end of the summer.

However, there has been no confirmation that any such trials have happened yet. 

Fears of torture

Ukraine, which has itself tried and convicted a string of Russian soldiers for war crimes committed against civilians, says the Azov captives are prisoners of war, deserving of protection under the Geneva Conventions.

The city of Mariupol was under siege for months as fighters from the Azov Regiment tried to push back Russian forces. After fighting for weeks from the bunkers and tunnels below Mariupol's vast steelworks, hundreds of Azov fighters surrendered in May.

Shamdasani said the OHCHR, which has dozens of monitors in Ukraine, has not been granted access to the prisoners and did not expect to be able to attend the planned trials.

"We are worried that denial of access to independent monitors leaves prisoners of war open to torture to extract confessions from them," she added.

Russia has denied torturing POWs, and say it is not committing any other forms of maltreatment against the Ukrainian captives.