Ukraine launched an enquiry on Sunday into what it said was an "execution" by Russia forces of two unarmed Ukrainian soldiers who had signalled their intention to surrender.
Ukrainian officials condemned the reported incident as a "war crime".
There was no official reaction from Moscow.
On Saturday a short video, whose authenticity could not be confirmed by AFP, was posted on Telegram showing two men coming out of a shelter, one with his hands above his head, before lying on the ground in front of another group of soldiers.
This is followed by what appears to be gunfire, and rising smoke, before the video cuts off abruptly.
The public prosecutor's office in the eastern Ukrainian region of Donetsk said that according to "preliminary data", the images were filmed near the village of Stepove, close to Avdiivka, an eastern town where fighting is raging.
"The video shows how a group of people in Russian uniforms shoot at close range two unarmed servicemen in the uniform of the Armed Forces of Ukraine who surrendered as prisoners," the prosecutor's office wrote.
"Investigators and prosecutors have started an investigation".
"The killing of prisoners of war is a gross violation of the Geneva Conventions and is classified as a serious international crime."
The Ukrainian army's strategic communications centre said on Saturday it possessed "confirmed information" that the video showed the "execution" by Russian forces of "unarmed soldiers".
Ukraine's human rights ombudsman, Dmytro Lubinets, condemned the incident as a "war crime".
Violating the rules of war
The Ukrainian soldiers "were disarmed and their hands were raised", Lubinets wrote on Telegram on Saturday.
"They did not pose any threat! The Russian side had to capture them and give them the status of prisoners of war."
The speaker of the Ukrainian parliament, Ruslan Stefanchuk, accused Russian forces of violating the rules of war.
The Ukrainian army urged the international community to hold Russia's military leadership "accountable".
In March, another video went viral, appearing to show a Ukrainian soldier being shot dead after shouting "glory to Ukraine".
At that time the United Nations High Commissioner for Human Rights, Volker Turk, said his office had documented numerous violations of international humanitarian law against prisoners of war.
These included "numerous summary executions and targeted attacks on civilians" by Russian forces and affiliated armed groups like the Wagner mercenary force, as well as "621 cases of enforced disappearances and arbitrary detention".