Pictures of cages to apparently contain Ukrainian prisoners of war during trials in Mariupol have been condemned by the UN.

Its human rights office said it was “concerned” they would not get a fair hearing and stressed that POWs cannot be prosecuted for lawful acts of war.

Images said to be from the philharmonic hall in occupied Mariupol – the city that was under siege for months before being captured by Russia – appear to show lattice-metal cages being erected.

‘We have information’ – US warning over Russian attacks in coming days – Ukraine latest

“We are concerned by reports that the Russian Federation and affiliated armed groups in Donetsk are planning – possibly in the coming days – to try Ukrainian prisoners of war in what is being labelled an ‘international tribunal’ in Mariupol,” said a spokesperson for the UN High Commissioner for Human Rights.

It said POWs have immunity and “cannot be prosecuted for having participated in hostilities, or for lawful acts of war committed in the course of the armed conflict”.

“If prisoners of war are charged with crimes, they are entitled to due process and fair trial guarantees,” said a statement.

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“No sentence or punishment may be passed on them unless it is delivered by an impartial and regularly constituted court.”

The Ukrainian defence ministry published the images and said a “show trial” was planned with “trained” witnesses.

It also claimed Russia was considering a “pinpoint missile attack” on the trial – which it would blame on Ukraine.

Concerns over the treatment and legal rights of captured fighters remain high.

Two Britons have already been given death sentences in separatist-controlled Donetsk after being accused of being mercenaries – charges they strongly deny. Three others face the same charges.

There’s also a risk of captured fighters being tortured to get a confession, said the UN human rights office, because there has been little independent monitoring of how they are being held.

It said Russian statements labelling Ukrainian POWs as “war criminals”, “Nazis” and “terrorists” also undermined their presumption of innocence.

Ukraine ‘special forces’ behind car bomb, says Russia

It comes amid a warning that Russia appears to be “stepping up efforts” to launch fresh attacks ahead of Ukrainian independence day on Wednesday.

A US intelligence official, speaking anonymously, said it is believed civilian infrastructure and government facilities would both be attacked.

The capital, Kyiv, has banned public celebrations commemorating independence from Soviet rule, while other regions have also restricted gatherings.

Meanwhile, Russia has blamed Ukrainian intelligence for the car bomb that killed the daughter of a leading right-wing political thinker over the weekend.

Darya Dugina, 29, died on the outskirts of Moscow, but her father – said to be influential on President Vladimir Putin – is believed to have been the intended target.

Russia’s federal security service, the FSB, said it was “prepared and perpetrated by the Ukrainian special services”.

It claimed a Ukrainian citizen, Natalya Vovk, carried out the killing and then fled to Estonia.

Ukraine has denied any involvement.