Experts commissioned by the UN’s top human rights body to look into violations in Ukraine said their initial investigation has turned up evidence of war crimes following Russia’s invasion of the country seven months ago.

Members of the Commission of Inquiry on Ukraine, mandated by the Human Rights Council earlier this year, have so far focused on four regions – Kyiv, Chernihiv, Kharkiv and Sumy.

Presenting their most extensive findings so far, they cited testimonies by former detainees of beatings, electric shocks and forced nudity in Russian detention facilities, and expressed grave concerns about executions in the four regions.

“We were struck by the large number of executions in the areas that we visited. The commission is currently investigating such deaths in 16 towns and settlements,” Erik Mose, the commission’s chairman, said.

He added that his team had received and was documenting “credible allegations regarding many more cases of executions”.

The investigators visited 27 towns and settlements, as well as graves and detention and torture centres, interviewed more than 150 victims and witnesses and met advocacy groups and government officials.

“Based on the evidence gathered by the commission, it has concluded that war crimes have been committed in Ukraine,” Mr Mose said.

He said the team had examined two incidents of ill-treatment against Russian soldiers by Ukrainian forces.

Mr Mose said an unspecified number of Russian soldiers were found to have committed crimes of sexual or gender-based violence – with victims ranging in age from four to 82 years old.

The commission plans to gradually expand its investigation, with areas of interest including allegations of filtration camps for people being detained or deported, the forced transfer of people, and allegations of expedited adoption of children.

During a 10-day trip to Ukraine in June, the team visited Bucha, a city outside Kyiv where Ukrainian authorities found mass graves and bodies strewn in the streets after Russian forces pulled out in late March.

The findings echo reports by news outlets and others of the destruction, death and despair in Ukraine since Russian President Vladimir Putin ordered the invasion on February 24.

The commission’s work could ultimately contribute to the work of International Criminal Court prosecutors who could bring charges over war crimes in Ukraine, although it remains uncertain whether Russia or other alleged perpetrators will ever face justice.

Anton Korynevych, ambassador-at-large for Ukraine’s Foreign Ministry, joined envoys from a number of Western countries who spoke out against Moscow’s war in the wake of the commission’s presentation. Russia’s delegation boycotted the council meeting.

Mr Korynevych, speaking by video, called for the creation of a special tribunal that would have jurisdiction “over the crime of aggression against Ukraine” and investigate senior Russian political and military leaders who were allegedly responsible.

He said accountability was crucial for rights violations and atrocities linked to Russia’s “aggression” but also highlighted how the effects of the war has rippled through the world and “put numerous countries on the verge of hunger, exacerbated extreme poverty, created the threat of nuclear catastrophe unseen before” and harmed the livelihoods of millions worldwide.

“The evidence of Russia’s atrocities becomes more horrifying by the day, most recently with the uncovering of mass graves in Izium, where the bodies show signs of torture,” Michele Taylor, the US ambassador to the Human Rights Council, said, referring to a Kharkiv region city that Ukrainian forces recaptured in recent weeks.

Ms Taylor urged the commissioners to continue to “examine the growing evidence of Russia’s filtration operations, forced deportations and disappearances”.

She referred to “numerous sources” that indicated Russian authorities have interrogated, detained and/or forcibly deported between 900,000 and 1.6 million Ukrainian citizens, and reports that children were being deported from Ukraine and placed in Russian orphanages for adoption.