The UN Human Rights Office says it is concerned by reports that Russia plans to try Ukrainian prisoners of war (POWs) in Russian-occupied Mariupol.

A UN spokeswoman said there was evidence that metal cages were being built in the Ukrainian city's concert hall, "apparently to restrain prisoners of war during proceedings."

The UN said prosecuting POWs for taking part in hostilities is a war crime.

Russia has previously denied unfairly treating prisoners of war.

Photos posted on social media in recent days -including by Ukrainian authorities- appear to show metal cages being built on the stage of the city's philharmonic hall.

The BBC has verified that the photos match the interior of the venue and were taken within the last four to five days.

Ukraine's intelligence service has alleged that Russia is planning to hold show trials of Ukrainian

POWs captured while defending the citybefore it fell to Russian forces in May.

Speaking at a press briefing, a spokeswoman for the UN High Commissioner for Human Rights, Ravina Shamdasani, said "apparently the idea is to restrain the prisoners of war during proceedings in these cages," adding that such plans were "not acceptable" and "humiliating".

"We recall that international humanitarian law prohibits the establishment of courts solely to judge prisoners of war and that wilfully depriving a prisoner of war of the rights of fair and regular trial amounts to a war crime," Ms Shamdasani said in a UN statement released on Tuesday.

She added that the UN is concerned that if POWs are charged with crimes, they will not receive a fair trial.

Ms Shamdasani also expressed concern over Russia denying Ukrainian POWs access to independent monitors such as the UN and the International Committee of the Red Cross (ICRC), which she said could expose them to the risk of being tortured to extract a confession. (BBC)