The U.N. human rights office expressed concern on Tuesday about plans by Russian-backed authorities to try Ukrainian prisoners of war (POWs) in the port city of Mariupol, possibly within days, saying such a process could itself amount to a war crime.

The Russian-backed authorities appear to be installing metal cages in a hall in Mariupol as part of plans to establish what they were calling an "international tribunal," Ravina Shamdasani, spokesperson for the Office of the U.N. High Commissioner for Human Rights (OHCHR), told a briefing.

"We are very concerned about the manner in which this is being done. 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," Shamdasani said, citing images on social media.