Foreign Secretary Liz Truss has said the death sentences handed to two Britons in Ukraine are "an egregious breach of the Geneva Convention".
Ms Truss said she had spoken with her Ukrainian counterpart to "discuss efforts to secure the release of prisoners of war held by Russian proxies".
"The UK continues to back Ukraine against Putin's barbaric invasion," she tweeted.
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Aiden Aslin, 28, and Shaun Pinner, 48, have been accused of being mercenaries after they were captured in Mariupol in April during the intense fight for control of the port city and later appeared in court in the separatist Donetsk People's Republic (DPR).
Prime Minister Boris Johnson was said to be "appalled" by the sentences handed to them and has ordered ministers to do "everything in their power" to secure their release.
A No 10 spokesperson said: "The prime minister was appalled at the sentencing of these men. He has been following the case closely and has asked ministers to do everything in their power to try and reunite them with their families as soon as we can.
"We completely condemn the sham sentencing of these men to death. There's no justification at all for this breach of the protection they're entitled to."
Asked if the UK government would talk to Russia to secure their release, the spokesperson said "we don't have regular interaction with the Russians".
They said the "priority" was to work with the Ukrainian government "to try and ensure their release as quickly as possible".
"They're afforded protection under the Geneva Convention as members of the Ukrainian Armed Forces, which is why we want to continue working with them closely to try and get them freed as quickly as we can."
Russian foreign minister Sergey Lavrov declined to comment on the cases, saying they are under the jurisdiction of the Donetsk People's Republic.
Mr Lavrov told a press conference: "Currently they are guided by the laws of the Donetsk People's Republic.
"Because these crimes were committed on the territory of the Donetsk People's Republic, all the rest is speculation.
"I will not comment on the Donetsk People's Republic judiciary."
Earlier on Friday, government minister Robin Walker said the government would use "all diplomatic channels" to raise the case of the two Britons.
He told Sky News: "As the foreign secretary has made clear, we will offer all support we can to them and their families.
"We have been absolutely clear throughout that these people should be treated as prisoners of war under the Geneva Convention, there is no basis on which they can be put on trial.
"This is an illegal court in a sham government that has held this trial and obviously we don't recognise it has any authority, but we will continue to use all diplomatic channels to make the case that these are prisoners of war who should be treated accordingly."
He added: "We utterly condemn the approach that's been taken here and we will use every method at our disposal to take this issue up."
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2022-06-10 11:15:00Z
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