A rebel court supported by Moscow sentenced two British men and a Moroccan citizen to death.
Aiden Aslin and Shaun Pinner, both British citizens, were captured fighting alongside Ukrainian forces.
Saaudun Brahim, a Moroccan national, was condemned for attempting to overthrow the government.
Ukraine is probe into the circumstances in which a rebel court supported by Moscow penalties two British men and a Moroccan citizen to death, according to Kyiv.
Aiden Aslin and Shaun Pinner, both British citizens, and Saaudun Brahim, a Moroccan national, were captured fighting alongside Ukrainian forces in the eastern Donetsk region and condemned by separatist authorities for attempting to overthrow the government.
Iryna Venediktova, Ukraine’s senior prosecutor, called the punishments “a violation and mockery” of international law and said the men should be treated fairly under Geneva treaties.
“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.
In April, Aslin and Pinner surrendered in Mariupol, a port city in southern Ukraine that Russian troops took in May following a weeks-long siege.
Since the beginning of Moscow’s February 24 invasion, Ukrainian courts have sentenced three Russian soldiers to hefty prison terms in war crimes cases.