Testimonies from former detainees revealed shocking accounts of human rights violations in the facilities, and expressed grave concerns about executions in the four regions.
The investigators of the Members of the Commission of Inquiry on Ukraine visited 27 towns and settlements in four of the worst hit regions of Ukraine: Kyiv, Chernihiv, Kharkiv and Sumy.
They also went to graves and detention and torture centres, interviewed more than 150 victims and witnesses and met advocacy groups and government officials.
“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.
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.
He added that his team had received and was documenting “credible allegations regarding many more cases of executions”.
“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.
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.
The report comes as Vladimir Putin ordered partial mobilisation in Russia, sparking a panic exodus of thousands of Russians who are fleeing for their lives into neighbouring countries.
According to Russian media reports, Moscow is looking to mobilise up to one million reservists for the Ukraine war as Putin makes desperate moves to take back the upper-hand in the conflict he began.