Children as young as four have been raped and tortured by Putin's thugs in Ukraine, UN experts have claimed.

The team from the Commission of Inquiry on Ukraine have investigated alleged war crimes in Kyiv, Chernihiv, Kharkiv and Sumy.

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.

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, Erik Mose, the commission's chairman, told the Human Rights Council.

He said: 'Based on the evidence gathered by the commission, it has concluded that war crimes have been committed in Ukraine.'

During a 10-day June trip to Ukraine, 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.

'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,' Mose said. 

He didn't specify who or which side in the war allegedly committed the killings.

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.

Commission member Pablo de Greiff told reporters the team had 'found two instances of ill-treatment of Russian Federation soldiers by Ukrainian soldiers. 

'We have found obviously significantly larger numbers of incidences that amount to war crimes on the part of the Russian Federation.