Ukrainian prosecutors said on Saturday they were investigating allegations Russian forces executed four Ukrainian prisoners of war and said a possible suspect for the killings was in custody.
"Prosecutors have initiated a pre-trial investigation... related to the deliberate killing of four prisoners of war," read a post on the Telegram channel of the prosecutor's office in the northeastern Kharkiv region.
"The investigation was initiated by interrogations of Russian prisoners of war, during which testimonies were obtained regarding the commission of the crime," read the post, citing chief prosecutor Oleksandr Filchakov.
The servicemen are alleged to have been killed on the orders of Russian military command over the summer at an aggregate plant in Vovchansk, which has been the focus of fierce fighting.
In May Russia launched an offensive into the Kharkiv region where Vovchansk is located -- an area they had first occupied and then withdrawn from at the start of the invasion in 2022.
"This case is unique as the person who may become a suspect in the said proceedings is also in Ukrainian captivity," Filchakov said.
If proven guilty of the war crime, the alleged perpetrator faces life imprisonment.
The Ukrainian prosecutor general's office said earlier this month it had information on the execution on the battlefield of "at least 92 Ukrainian soldiers who had surrendered".
This included allegations that Russian forces executed 16 POWs near the eastern city of Pokrovsk, a key logistics hub for the Ukrainian army.
Russia and Ukraine have repeatedly accused each other of killing POWs since Moscow launched its invasion of Ukraine in February 2022.