The court in Donetsk, one of two self-proclaimed statelets in eastern Ukraine, last week ordered the death penalty for the trio after they were captured by Russian troops.

Brahim Saadoun along with UK nationals Aiden Aslin and Shaun Pinner were said to have surrendered in April after fighting with Ukrainian forces in the besieged port city of Mariupol.

British cabinet minister Brandon Lewis told Sky News the government was "fully engaged" with Ukrainian authorities in trying to help Aslin and Pinner after their "sham trial".

The Britons were legal combatants serving with Ukraine's armed forces and fully entitled to protection for prisoners of war under the Geneva convention, Lewis said.

The UK should also intervene on Saadoun's behalf, his friend Zina Kotenko told Sky from her new home in northern England, after fleeing Russia's invasion.

Kotenko said she had first met the 21-year-old Saadoun in a Kyiv nightclub, described him as a "kind", "open-minded" and "cheerful" person.

Kotenko said he had been accepted into the Ukrainian army after several attempts, being found previously by recruiters to be underweight.

"Please care about people who care about democracy," she urged the UK government.

"People are the voice, people are the face of the government, now the face of the government is sitting in prison... please save (Saadoun)."

Morocco's government has not commented on the case. Saadoun's father has told local media that his son was a student in Ukraine before legitimately signing up to fight.

Dmytro Khrabstov, 20, is another friend who met Saadoun on Kyiv's pre-war party scene and is campaigning for his release with the social media hashtag #SaveBrahim.

The Moroccan joined the Ukrainian military last year, telling friends he wanted to "die as a hero", according to Khrabstov.

He is a "bright and enthusiastic guy, dreaming about the technology of the future and how he could change things", the friend told Britain's PA news agency.

Both the Britons have been living in Ukraine since 2018 and signed up to fight when Russia invaded, according to their families, denying that they were mercenaries as alleged by the court.

Pinner, 48, is married to a Ukrainian woman, and Aslin, 28, is engaged to a local.

Pinner's family on Saturday said they were "devastated" at the court's verdict, and that "our hearts go out to all the families involved in this awful situation".

"We sincerely hope that all parties will cooperate urgently to ensure the safe release or exchange of Shaun," they said.

Ukraine's ambassador in London, Vadym Prystaiko, said Friday that the British pair were likely to be traded for pro-Russian lawmakers held by Ukraine.

"It will be a swap," he told the BBC. "The important question is what will be the price for this."