Russian citizen Arseniy Pavlov, known as Motorola, claims to have killed 15 Ukrainian prisoners.

"The new evidence of these summary killings confirms what we have suspected for a long time. The question now is: what are the separatist leaders going to do about it?" said Denis Krivosheev, Europe and Central Asia Deputy Director at Amnesty International.

Footage reviewed by Amnesty International shows Ukrainian soldier Ihor Branovytsky, one of the defenders of Donetsk airport, being taken captive and interrogated. The video, posted on YouTube, shows signs that he was hit in the face. He remained in captivity until he was killed.

"The torture, ill-treatment and killing of captured, surrendered or wounded soldiers are war crimes. These claims must be promptly, thoroughly and impartially investigated, and the perpetrators prosecuted in fair trials by recognized authorities," Krivosheev said.

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A number of individuals claim to have seen Ihor Branovytsky being shot and killed by a separatist commander. His body was returned to his family earlier this month and he was buried in Kyiv on 3 April. The Ukrainian security services have opened an investigation into his killing.

Amnesty International has also seen videos documenting the captivity, and pictures of the dead bodies, of at least three other members of the Ukrainian armed forces, reportedly being held in a morgue in Donetsk. There are signs of bullet wounds to their heads and upper parts of their bodies, apparently the result of execution-style killings. The soldiers had been captured by pro-Russian forces in Debaltseve between February 12 and 18, 2015 when the defending Ukrainian forces were encircled there.

The revelation follows a report by the Ukrainian newspaper Kyiv Post on 6 April featuring a phone interview, allegedly withy Arseniy Pavlov, better known by his nom-de-guerre “Motorola.” Pavlov, reportedly a Russian national and the leader of the pro-Russian armed group known as the Sparta Battalion, which is operating in eastern Ukraine, claimed he had "shot dead" 15 soldiers captured from the Ukrainian armed forces. He is alleged to be the killer of Ihor Branovytsky.