Amnesty International has said it has evidence pro-Russian rebels in eastern Ukraine summarily killed four Ukrainian soldiers in their captivity.
Witnesses told the group that one government soldier was shot at point-blank range by a separatist commander.
Amnesty also said it had videos of three others shown alive in captivity, then dead in a morgue with bullet wounds to their heads and upper bodies.
A fragile ceasefire has been in force in eastern Ukraine since February.
The killings are tantamount to war crimes, Amnesty said.
“These claims must be promptly, thoroughly and impartially investigated, and the perpetrators prosecuted in fair trials by recognized authorities,” said Denis Krivosheev, Europe and Central Asia Deputy Director at the organisation.
The allegations come after one separatist commander told the Ukrainian newspaper Kyiv Post he personally killed 15 soldiers captured from the Ukrainian armed forces.
An adviser to the rebels told the BBC last year they had carried out executions in one separatist-held area “to prevent chaos”.
Rights groups have accused both sides in the conflict of abuses.
Some 6,000 people have been killed since fighting broke out in eastern Ukraine between pro-Russian separatists and the Ukrainian government a year ago.