"You will know that under the Geneva Convention prisoners of war are entitled to combatant immunity and they should not be prosecuted for participating in hostilities," he said.
"So we will continue to work with the Ukrainian authorities to try and secure the release of every British citizen who is serving in the Ukrainian armed forces and who is being held as a prisoner of war."
A video published by the DPR Supreme Court shows Aslin giving his defense from behind bars, including Pinner and Brahim sitting next to him.
Secretary of State Liz Truss condemned the sentence as a false judgment that has absolutely no legitimacy.
“My thoughts are with the family. We continue to do everything we can to support them," she said.
The shocking court ruling comes after prisoners of war were led to believe that a maximum sentence of 40 years behind bars was the most likely sentence.
The three men were arrested in Mariupol in mid-April, with Pinner and Aslin along with fellow Briton Andrew Hill, 35, paraded in front of Russian TV cameras in violation of the Geneva Conventions.