Kyiv said on Thursday that Russian forces had executed four captured Ukrainian servicemen in the eastern Donetsk region, the latest war crime allegation since Moscow invaded Ukraine nearly three years ago.
Both Moscow and Kyiv have accused each others' soldiers of executing prisoners of war.
The Ukrainian prosecutor general's office said four servicemen fighting in the national guard had been captured on October 6 near the Donetsk region town of Selydove, where Russian forces are advancing.
"The next day, the Defence Forces regained their lost positions and found the bodies of the killed Ukrainian servicemen during a search operation," the prosecutor general said in a statement.
There was no immediate response to the claims from Moscow and AFP could not independently verify the allegations.
"The killing of prisoners of war is a gross violation of the Geneva Conventions and is classified as a serious international crime," said the statement from Kyiv, adding that a war crime probe into the incident had been launched.
Ukrainian authorities said this month that Russian forces had killed more than 100 prisoners of war in Russian custody.
In March 2023 a video showing an captured Ukrainian soldier shouting "Glory to Ukraine!" moments before being executed by shooting squad, went viral.
The United Nations says it has documented numerous violations of international humanitarian law against prisoners of war, including cases of summary execution of both Russian and Ukrainian prisoners.