“This disgusting Soviet-era-style show trial is the latest reminder of the depravity of Putin’s regime,” he wrote, adding: “They cannot treat British citizens like this and get away with it.”

Under the Geneva Conventions, prisoners of war must be treated humanely and be protected from violence, intimidation, insults and public curiosity, as well as sheltered and provided with food, clothing and medical care.

Denis Krivosheev, an official with Amnesty International, said that the sentences were a “blatant violation of international humanitarian law.”

“The three were members of the Ukrainian regular forces,” he said, “and under the Geneva Conventions, as prisoners of war, they are protected from prosecution for taking part in hostilities.” The only exception, he said, is prosecutions on war crimes charges.

According to the BBC, Mr. Aslin moved to Ukraine in 2018 and joined its military. He is engaged to a Ukrainian woman, the broadcaster said. Mr. Pinner comes from Bedfordshire, had served in the British Army and married a Ukrainian, the BBC reported.

Mr. Saadoun arrived in Ukraine in 2019, learned Russian, and signed up for the Ukrainian army a year ago, a friend, Ilya Zub, said.

“Brahim is not a mercenary,” Mr. Zub said, adding that he had known Mr. Saadoun for more than a year. “He came to Ukraine in 2019 and decided he wanted to start a new life.”