Three men are at risk of facing the death penalty after they were captured by Russian forces while fighting for Ukraine.
Britain has condemned what it calls the exploitation of prisoners of war for political purposes.
Here's what we know so far, and what might happen next.
Who are the three men imprisoned?
The three prisoners include Britons Aiden Aslin and Shaun Pinner, and Moroccan Brahim Saadoun.
Mr Aslin, 28, and Mr Pinner, 48, were captured in April while defending the south-eastern city of Mariupol.
According to the BBC, Mr Aslin has a Ukrainian fiancée and now has dual citizenship, after moving to Ukraine in 2018 and becoming a marine in the country's military.
Mr Aslin also previously fought with Kurdish armed units in Syria against the Islamic State group.
Mr Pinner was a "well-respected" soldier in the British Army before he moved to Ukraine four years ago to join the Ukrainian military, his family said in a statement.
They added that Mr Pinner's three-year contract was due to expire at the end of the year and he was planning to enter a humanitarian role in the country.
Mr Saadoun was also captured in April.
Why were they captured?
According to Russian media outlet Tass, the three men are accused of being "mercenaries" and "participating in hostilities as part of the Ukrainian armed formations".
Russian media also claims Mr Pinner learned about the possibility of fighting on the side of Ukraine as a mercenary from Mr Aslin, as did Mr Saadoun.
Following their capture in April, the two Britons were shown on Russian TV asking to be freed in exchange for a Ukrainian ally of Russian President Vladimir Putin who had been detained by Ukrainian authorities.
"If Boris Johnson really does care like he says he does about British citizens, then he would help pressure [Ukrainian President Volodymyr] Zelenskyy to do the right thing and return Viktor [Medvedchuk] to his family — and return us to our families," Mr Aslin says in the video, while an unidentified man stands over him.
In the footage, both men appeared to have bruises on their face, sparking calls from their families for the Geneva Convention to be respected.
The Geneva Convention states that prisoners of war must be protected "against acts of violence or intimidation and against insults and public curiosity".
Russian foreign ministry spokeswoman Maria Zakharova said: "Don't worry, the Russian side is taking care of them.
"They are fed, watered, and given the necessary assistance. Just like other foreigners who have surrendered or been detained," she said.
The BBC reported that Ms Zakharova also referenced what she described as a call by Prime Minister Boris Johnson for British detainees to be shown mercy, and said that in turn, Britain should "show mercy" to the citizens of Ukraine by stopping the supply of weapons to the Ukrainian government.
Mr Johnson had urged Russia to treat captured British fighters "humanely and compassionately".
What happened in court?
The trial is taking place in the Donetsk People's Republic, one of two breakaway Russian-backed entities in the Donbas region of eastern Ukraine which Russia says it is fighting to "liberate" from Ukrainian forces.
Three days before launching its February 24 invasion of Ukraine, Russia recognised them as independent states in a move condemned by Ukraine and the West as illegal.
Video published by Russian media agency RIA showed Mr Aslin, Mr Pinner and Mr Saadoun in a courtroom cage with white bars.
RIA said Mr Pinner and Mr Saadoun had pleaded guilty to actions aimed at the violent seizure of power.
The video appeared to show Mr Aslin pleading guilty to a lesser charge involving weapons and explosives.
He was seen standing in the cage and leafing through a sheaf of legal documents as the charge was translated to him.
The news agency quoted prosecutors as saying the combined charges could mean the death penalty for all three.
What has Britain said in response?
Britain's Foreign, Commonwealth and Development Office condemned what it called the exploitation of prisoners of war for political purposes.
"They are entitled to combatant immunity and should not be prosecuted for participation in hostilities," said a spokesperson late on Wednesday.
Justice Secretary Dominic Raab said he "expected the laws of armed conflict to be respected" and the Foreign Office would be "looking at making sure all those representations are made."
Conservative MP and former housing secretary Robert Jenrick said Mr Aslin should be returned to Ukraine at the earliest opportunity, potentially through a prisoner exchange, and condemned the "trumped-up charges" faced by both Britons.
"This is a British citizen, but who also holds Ukrainian nationality, is married to a Ukrainian, joined the Ukrainian armed forces in the normal way prior to [Vladimir] Putin's illegal invasion, and has been serving in the armed forces," he told the BBC.
"He was taken prisoner by Russian forces and in accordance with international law and the Geneva convention, he should be being held appropriately and returned to Ukraine at the earliest possible opportunity, possibly through a prisoner exchange.
"Instead of that, Putin's regime have chosen to put him and another British national, Shaun Pinner, on trial on trumped up charges, no evidence whatsoever.
What might happen next?
It is feared the court, which is not internationally recognised, could pass the death penalty if it convicts them.
Mr Jenrick told the BBC in April that a prisoner exchange was the "most likely route" to get the men to safety.
But he added it would be for the Ukrainian government to decide whether a swap could take place.
ABC/wires
