London — The British government said Thursday that it was "deeply concerned" that two U.K. nationals captured while fighting with the Ukrainian military had been sentenced to death. Prime Minister Boris Johnson's spokesman Jamie Davies said that under the Geneva Conventions, prisoners of war are entitled to combatant immunity and "shouldn't be exploited for political purposes."

    He said: "We will continue to work with Ukrainian authorities to secure the release of any British nationals who were serving in the Ukrainian armed forces and are being held as prisoners of war."British Foreign Secretary Luz Truss tweeted: "I utterly condemn the sentencing of Aiden Aslin and Shaun Pinner held by Russian proxies in eastern Ukraine. They are prisoners of war. This is a sham judgment with absolutely no legitimacy."

    A court in the self-proclaimed Donetsk People's Republic found Aslin and Pinner, as well as Saaudun Brahim of Morocco, guilty of taking action toward a violent overthrow of power, an offense punishable by death in the internationally unrecognized rebel-held territory in Ukraine's Donbas region.

     Russian state news agency RIA Novosti reported Thursday that the three would face a firing squad. They have a month to appeal to a higher court, however.

   The three men fought alongside Ukrainian troops and surrendered to Russian forces weeks ago.

CBS News' partner network BBC News said Aslin, 28, and Pinner, 48, were both from England but had lived in Ukraine since 2018. Aslin is engaged to a Ukrainian woman, the BBC said. Their families told the British network that they had been fighting in the Ukrainian military when they were captured in April. 

   People from around the world have traveled to fight alongside Ukraine's forces since Russia launched its invasion on February 24, and that includes Americans. Ukrainian officials have estimated that some 4,000 people, many with previous military experience, came to join the fight from the U.S.

   In May, CBS News correspondent Debora Patta met some of the American volunteers who have been among both the battlefield casualties, and the heroes to emerge from the war.