The three men - Britons Aiden Aslin and Shaun Pinner, and Moroccan Saaudun Brahim – were captured while fighting for Ukraine, reportedly defending the southern port city of Mariupol.

Bitter fighting between Ukrainian and Russian forces since the Russian invasion on 24 February flattened the city, where UN rights chief Michelle Bachelet has previously condemned attacks against civilians and civilian infrastructure, that have likely caused thousands of deaths.

“OHCHR is concerned about the so-called Supreme Court of the self-proclaimed Donetsk People’s Republic sentencing three servicemen to death,” said Ms. Shamdasani. “According to the chief command of Ukraine, all the men were part of the Ukrainian armed forces and if that is the case, they should not be considered as mercenaries.”

Answering a question at the regular briefing in New York on Thursday about the death sentences handed down, the UN Spokesperson Stéphane Dujarric, said the the Organization always has "and we always will", opposed the death penalty under any circumstances. "And we would call on the combatants who have been detained, to be afforded international protection, and to be treated according to the Geneva Conventions", he added.

Longstanding concerns

The UN rights office spokesperson also highlighted longstanding concerns about fair trial violations in Ukraine’s breakaway eastern regions bordering Russia. “Since 2015, we have observed that the so-called judiciary within these self-contained republics has not complied with essential fair trial guarantees, such as public hearings, independence, impartiality of the courts and the right not to be compelled to testify.”

Speaking in Geneva, Ms. Shamdasani added that “such trials against prisoners of war amount to a war crime. In the case of the use of the death penalty, fair trial guarantees are of course all the more important.”