In a punishment handed down by the country’s pro-Moscow rebels, two British citizens and a Moroccan were on Thursday sentenced to death for fighting on Ukraine’s side.

A court in the self-proclaimed Donetsk People’s Republic found the three men guilty of working toward a violent overthrow of power, an offence punishable by death in the unrecognised eastern republic. They were also convicted of mercenary activities and terrorism.

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Russia’s state news agency RIA Novosti reported that the three — Aiden Aslin, Shaun Pinner and Saaudun Brahim — are set to face a firing squad. They have a month to appeal.

The separatists argued that the three fighters were “mercenaries” not entitled to the usual protections afforded prisoners of war. They are the first foreign fighters to be sentenced by Ukraine’s Russian-backed separatists.

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Aslin’s and Pinner’s families had contended that the men, who are both said to have lived in Ukraine since 2018, were “long-serving” members of the Ukrainian military.

British Foreign Secretary Luz Truss condemned the sentencing as a “sham judgement with absolutely no legitimacy.” British Prime Minister Boris Johnson’s spokesman Jamie Davies said that under the Geneva Conventions, POWs are entitled to immunity as combatants.

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The three men fought alongside Ukrainian troops. Pinner and Aslin surrendered to pro-Russian forces in the southern port of Mariupol in mid-April, while Brahim did so in mid-March in the eastern city of Volnovakha.

It is reported that the Russian military argued foreign mercenaries fighting on Ukraine’s side are not combatants and should expect a long prison term, at best, if captured. Another British fighter captured by the pro-Russian forces, Andrew Hill, is awaiting trial.

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Meanwhile, Russian forces pounded the eastern city of Severodonetsk in fierce, street-by-street combat that Ukrainian President Volodymyr Zelenskyy said could determine the fate of the Donbas, the country’s industrial heartland of coal mines and factories.

Moscow-backed separatists have been fighting Ukrainian troops for years in the Donbas and held swaths of territory before the invasion.