Amnesty International says it has new evidence of "execution-style killings" by pro-Russian separatists in eastern Ukraine.
The London-based human rights organization said on April 9 it had photographs of three dead Ukrainian soldiers with bullet wounds to their heads and upper bodies.
It said earlier footage posted on YouTube shows the same men alive in the captivity of rebels.
They are thought to have been detained in the town of Debaltseve between February 12 and 18 as separatist militants encircled Ukrainian forces there.
According to Amnesty International, the bodies are held at a morgue in Donetsk.
The group said it was not releasing the soldiers' names in order to protect their sources in Ukraine.
It also quoted witnesses as saying that another soldier, identified as Ihor Branovytskiy, was shot at point-blank range by a separatist commander, Arseny Pavlov.
Branovytskiy was reportedly buried in Kyiv in April 3.
Pavlov, who is better known by his nom de guerre "Motorola", declined to comment when contacted by Reuters.
Amnesty International's deputy director for Europe and Central Asia, Denis Krivosheyev, said the killing of captives amounts to war crimes.
"Anyone being ill-treated or killed in custody constitutes a war crime," he told RFE/RL. "Those responsible for this should be identified and prosecuted."
Krivosheyev said ill-treatment and torture of captives was "a widespread practice" on both sides of the conflict in eastern Ukraine.
While evidence of summary executions is rare, Krivosheyev said he suspected "a lot more is going on than what we know."
Amnesty International's allegations come after a rebel commander told the newspaper Kyiv Post he had killed 15 captured Ukrainian soldiers.
In Kyiv, Ukrainian state security official Vasyl Vovk said government authorities would try to put Pavlov on an international wanted list.
"This person will bear responsibility for crimes against humanity, torture, murder of Ukrainian citizens including servicemen of the Ukrainian armed forces," he said.
In Washington, U.S. State Department spokesman Jeffrey Rathke said the United States was "deeply troubled" by the Amnesty report.
"These serious accusations must be thoroughly and transparently investigated. And any perpetrators must be held to account," Rathke said.
A spokeswoman for the separatists, Daria Morovova, denied the allegations in an interview with the AFP news agency.
More than 6,000 people have been killed in the conflict between government forces and Russian-backed rebels in eastern Ukraine over the past year.
Rights groups have accused both sides of abuses.