The United Nations said Friday it was "deeply concerned" by what it said were summary executions of prisoners of war by both Russian and Ukrainian forces on the battlefield.

The allegations came shortly after Kyiv accused Russian forces of killing a captured Ukrainian serviceman who was filmed saying "Glory to Ukraine" before being shot dead.

The head of the UN Human Rights Monitoring Mission in Ukraine, Matilda Bogner, said that her organisation had recently recorded killings by both sides.

"We are deeply concerned about (the) summary execution of up to 25 Russian prisoners of war and persons hors de combat by the Ukrainian armed forces, which we have documented," Bogner said at a press conference in Kyiv on Friday.

"This was often perpetrated immediately upon capture on the battlefield," she said.

"While we are aware of ongoing investigations by Ukraine authorities into five cases involving 22 victims, we are not aware of any prosecution of the perpetrators," she added.

Bogner also expressed "deep" concern over the alleged executions of 15 Ukrainian prisoners by Russian armed forces after their capture.

She said the Wagner mercenary group, which claims to be leading Russia's assault for Bakhmut -- the longest and bloodiest battle of the war -- was responsible for 11 of those killings.

Moscow and Kyiv have accused each other of mistreating prisoners of war since Russian President Vladimir Putin invaded a year ago.

In response to the UN report, Kyiv's Ministry of Foreign Affairs said it was "unacceptable" to hold the "victim of aggression" responsible.

- 'They broke us' -
One UN report issued Friday claimed Ukrainian military personnel had subjected prisoners of war to death threats, mock executions or threats of sexual violence. Some beatings were "purely retaliatory", it said.

"In some cases, officers beat POWs saying: 'This is for Bucha'," the mission reported detainees as saying, referring to a town near Kyiv where Russian forces were accused of widespread atrocities.

"Before questioning, they showed me an axe handle covered in blood as a warning," the report quoted a Russian POW as saying.

"The questioning lasted for about an hour and they used electricity six times, whenever they thought I was lying," the detainee said, according to the report.

Ukrainian POWs quoted in the report said they were subjected to torture, sexual violence, a lack of food and water and denied medical attention.

They said they were tortured and ill-treated to extract information or as a form of punishment, the mission said.

Ukrainian prisoners reported being beaten with shovels, stabbed, subjected to electric shocks and strangled.

"Some of them lost their teeth or fingers, had their ribs, fingers or noses broken," the report said.

"They did not just beat us, they broke us. They used their fists, legs, batons, tasers. There were POWs who had their arms or legs broken," one man was quoted as saying.

The Ukrainian parliament's human rights commissioner Dmytro Lubinets said Friday that he was "surprised" by the allegations against Ukrainian troops and said he had not been informed of them in advance.

On Telegram, he wrote that he wanted to "know the facts and the indisputable arguments on which the conclusions" of the UN report were based.

In a separate statement on Friday, Kyiv's Ministry of Foreign Affairs thanked the UN monitors for their work but emphasised that Ukraine "expects that the UN mission will avoid any steps that may be interpreted as equalising the victim and the aggressor".