Pro-Russia separatists in Donbass, eastern Ukraine, have carried out summary executions of Ukrainian soldiers they have captured, according to Amnesty International.
In a statement posted on Thursday, the rights group, said it had reviewed footage purporting to show a Ukrainian soldier with signs of abuse on his face, and who kept in captivity until he was killed.
The organisation says a number of people say they saw Ihor Branovytsky being shot dead at point-blank range by a rebel commander.
His body was returned to Kiev earlier in April, and Ukrainian security servives have since started an investigation.
Amnesty International also says it has seen video and photographic evidence showing bodies of Ukrainian troops with bullet wounds to their heads and upper bodies, apparently a result of executions.
The soldiers had been captured by pro-Russian forces in Debaltseve in February 2015 when the defending Ukrainian forces were encircled there, according to the group.
The statement by the organisation follows a report that appeared in Ukrainian newspaper Kyiv Post on April 6, in which a rebel soldier, Arseniy Pavlov, better known by his nom-de-guerre “Motorola”, reportedly admitted shooting dead 15 captured Ukrainian soldiers.
Pavlov, who is believed to be a Russian national, is alleged to have killed Ihor Branovytsky.
"Summary killings are a war crime, plain and simple. The leaders of the self-styled ‘Donetsk People's Republic’ in eastern Ukraine must send their members a clear message: those who fight with them or on their behalf must respect the laws of war, " said Denis Krivosheev, Europe and Central Asia deputy director at Amnesty International.
"These claims must be promptly, thoroughly and impartially investigated, and the perpetrators prosecuted in fair trials by recognised authorities."
Source: Al Jazeera