Amnesty International on Thursday said it had new proof of the "summary killings" of Ukrainian soldiers captured by pro-Russian separatist fighters in eastern Ukraine, and demanded a quick impartial probe.
The rights group notably cited the case of Ukrainian soldier Igor Branovytsky, who was taken prisoner during fighting at Donetsk airport, saying he was tortured and killed while in captivity.
"Video, posted on YouTube, shows signs that he was hit in the face. He remained in captivity until he was killed," Amnesty said in a statement, adding that a number of people claimed to have seen him "being shot and killed point-blank by a separatist commander".
His body was returned to his family in early April and he was buried in Kiev on April 3. The Ukrainian security services have opened an investigation.
Amnesty said it had also seen videos with pictures of the dead bodies of at least three other members of the Ukrainian armed forces, with signs of bullet wounds to the head and upper parts of their bodies, apparently the result of execution-style killings.
They were captured by pro-Russian forces in Debaltseve between February 12 and 18 when the Ukrainian army was encircled there.
Amnesty also cited a telephone interview published by the English-speaking Kyiv Post on April 6 with a prominent rebel commander known by his nom-de-guerre of "Motorola" who claimed to have "shot dead" 15 captive Ukrainian soldiers.
"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," said Amnesty's Europe and Central Asia deputy director Denis Krivosheev.
In Ukraine, separatist spokeswoman Daria Morozova told AFP that the rebels "refute rumours prisoners of war were shot in the DNR (self-proclaimed Donetsk People's Republic)."
"These facts did not take place," she added. "We have no more Ukrainian prisoners of war."
State Department spokesman Jeffrey Rathke said the United States was "deeply troubled" by the reports.
"These serious accusations must be thoroughly and transparently investigated. And any perpetrators must be held to account," Rathke said.
More than 6,000 people have been killed in fighting between the separatists rebels and the Ukraine authorities that began a year ago this week.