Amnesty International released a statement Thursday, condemning the “execution-style killings” conducted by pro-Russian rebels in the conflict-torn region of Donbass in eastern Ukraine. The international organization, which tracks human rights, said that the killings should be investigated at the earliest.

The organization also said the rebels killed four Ukrainian soldiers in captivity. Witnesses to the incident, according to Amnesty, said that one Ukrainian soldier was shot at point-blank range by a separatist commander. The soldier, identified as Ihor Branovytsky, was one of those defending the Donetsk airport. He was reportedly taken captive, interrogated and later hit in the face, leading to an investigation by the Ukrainian security forces into his killing. 

Amnesty said it had videos of three other soldiers kidnapped in February, who were alive in captivity and later seen in a morgue with gunshot wounds on their heads and upper parts of their bodies, signaling an execution-style killing.

“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?” Denis Krivosheev, the deputy director for Europe and Central Asia at Amnesty International, said in the statement, adding: “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.”

The Amnesty report comes as Arseniy Pavlov, who is believed to be the leader of the pro-Russian armed group known as “Sparta Battalion,” said in a telephone conversation with Kyiv Post, a local newspaper, that he killed 15 prisoners of war.

“I don’t give a f**** about what I am accused of, believe it or not,” Pavlov, who is better known as Motorola, said according to the Kyiv Post article published Monday. “I shot 15 prisoners dead. I don’t give a f****. No comment. I kill if I want to. I don’t if I don’t."

Krivosheev referred to Pavlov's confession in the Amnesty report, saying: “This chilling ‘confession’ from a separatist fighter, alongside video evidence and testimony from witnesses, and the mounting evidence of abuses of captives by both sides, highlights the urgent need for an independent investigation into this and all other allegations of abuses in this conflict which began a year ago."

“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. They must urgently remove from their ranks anyone suspected of responsibility for ordering or committing serious violations of international humanitarian law and human rights abuses, and fully cooperate with any independent investigation,” Krivosheev said in the statement.

According to an estimate by the United Nations, about 6,000 people have been killed in clashes that started a year ago between pro-Russian rebels and Ukraine's government forces in eastern Ukraine.