Ukrainian authorities said on Sunday they had opened an investigation into alleged shootings by Russian forces of six unarmed Ukrainian soldiers in the city of Avdiivka, and two at a village in the same region, after Russia claimed full control of the city.

"An investigation into the shootings of unarmed Ukrainian prisoners of war in Avdiivka and Vesele has been launched," the prosecutor's office in Donetsk region said on its Telegram channel.

The prosecutors referred to information about "the execution of six prisoners" at a position in Avdiivka, adding that they were "seriously injured and awaiting evacuation." It said the information had been shared on Telegram but did not name the source.

Ukrainian public broadcaster Suspilne quoted Ukrainian military spokesperson Dmytro Lykhoviy as saying: "There is still no possibility to confirm or refute" the killings at Avdiivka, as an investigation was ongoing. However, he added it would not be the first time Russia had committed war crimes.

The Donetsk prosecutor's office said a video fragment taken by a drone showed a Russian soldier shooting two captured Ukrainian soldiers at close range at the village of Vesele.

"Not wanting to leave the prisoners alive, the occupier kills them with automatic weapons," the office added, without saying when the incident had occurred.

Earlier on Sunday, the official Telegram channel of Ukraine's land forces shared video apparently shot from a drone of people in uniform in a narrow trench.

An armed person the post identifies as Russian is seen approaching two uniformed people identified as Ukrainian and then discharging his weapon and apparently shooting them both.

The Ukrainians in the footage do not appear to be attempting to defend themselves and are in a narrow trench in single file as the other soldier approaches and appears to grab them both.

After initial confrontations, the two injured people are seen collapsing on to their sides in the trench, twitching. The other person is then seen standing back briefly, before appearing to shoot them both again.

Russia's defense ministry did not respond to a Reuters email sent outside regular business hours seeking comment on the allegations, the latest in a series of accusations leveled by Ukrainian prosecutors that it has killed prisoners of war. Russia has denied such claims.

Reuters could not immediately verify the video or other details of the account by the Ukrainian prosecutors and military.