A Russian commander in the Kremlin-backed separatist battalion in eastern Ukraine claims to have shot dead 15 Ukrainian soldiers taken captive by his fighters.

Meanwhile, Amnesty International discovered evidence of summary killings of Ukrainian soldiers at the hands of separatists.

In a telephone interview with the Kyiv Post on April 3, Arseny Pavlov, commander of the separatists' Sparta Battalion, said he had killed the troops when asked about accounts from witnesses who claim he executed Ihor Branovytsky, a Ukrainian soldier. Branovytsky fought in the battle for Donetsk Airport before surrendering, and being taken prisoner by the separatists.

"I don't give a f*ck about what I am accused of, believe it or not," Pavlov, better known by his nom de guerre, "Motorola," is heard saying on the recorded phone conversation. "I shot 15 prisoners dead. I don't give a f*ck. No comment. I kill if I want to. I don't if I don't."

Mashable cannot verify the authenticity of the recording.

Several witnesses claim to have seen Branovytsky being shot and killed point-blank by a separatist commander. His body was returned to his family this month, and he was buried in Kiev on April 3.

The Ukrainian security services have opened an investigation into Branovytsky's killing, and said they are looking into Pavlov's claims of killing 15 government troops.

The separatist commander's apparent confession came days before Amnesty International uncovered evidence of "execution-style killings" of Ukrainian soldiers by pro-Russian armed groups in eastern Ukraine.

Amnesty said it has videos documenting the captivity, and photos 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," according to the organization's report.

The soldiers were reportedly captured by separatist forces during the battle for Debaltseve between Feb. 12 and 18.

Igor Strelkov, a former pro-Russian separatist commander, second left, attends the wedding ceremony of Arseny "Motorola" Pavlov and Elena Kolenkina in Donetsk, eastern Ukraine, Friday, July 11, 2014.

Pavlov climbed the rebel ranks after his mentor, infamous former Russian rebel leader Igor "Strelkov" Girkin, was called back to Moscow last July. Girkin, who said he "started the war in Ukraine," has also been accused of summary killings.

Mashable recovered documents in Girkin's burnt-out office in Sloviansk, eastern Ukraine, after his forces retreated from the city to the rebel-held capital of Donetsk on July 5. The documents outline three summary executions by firing squad of local men, who were convicted under Stalin-era laws by a makeshift rebel military tribunal for stealing and kidnapping.

The abandoned office of Igor Strelkov, commander of the self-styled Donetsk People's Republic. The execution orders were found in this building. Image: Evgeny Feldman, Mashable

"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," Denis Krivosheev, Europe and Central Asia deputy director at Amnesty International, said of the alleged Pavlov confession tape and evidence of other summary killings.

"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."