Ukraine is investigating the circumstances in which a Moscow-backed separatist court sentenced two British men and a Moroccan citizen to death, Kyiv said.
British citizens Aiden Aslin and Shaun Pinner and Saaudun Brahim from Morocco were captured fighting with Ukrainian forces and sentenced by separatist authorities in the eastern Donetsk region for attempting to overthrow the government.
Ukraine's chief prosecutor Iryna Venediktova late Thursday described the sentences as "a violation and mockery" of international law and said the men should be treated fairly under Geneva conventions.
"Ukraine has already initiated a pre-trial investigation into this matter and will take all appropriate steps to ensure that everyone that was involved in this illegal action is held responsible for their actions," she said in a statement.
Aslin and Pinner had surrendered in April in Mariupol, a port city in southern Ukraine that was captured by Russian troops in May after a weeks-long siege.
Ukrainian courts have handed three Russian soldiers long prison sentences at war crimes trials since the start of Moscow's February 24 invasion.
Venediktova said Thursday that Ukraine is investigating over 100 Russian soldiers for suspected war crimes.
