"Their rights have been undermined in every aspect of life, leaving deep scars, both physical and psychological," OHCHR spokesperson Liz Throssell said at a briefing in Geneva, reading a statement by U.N. human rights chief Volker Turk.

The Russian Mission in Geneva did not respond to a request for comment when contacted by Reuters.

"In the four regions of Ukraine that were illegally annexed by the Russian Federation in 2022, children have been particularly affected by violations of international human rights law...including summary executions, arbitrary detention, conflict-related sexual violence, torture and ill-treatment," the report said.

Five boys and two girls were summarily executed in 2022 and 2023, with the report noting that the wilful killing of civilians was a war crime and a grave breach of the Geneva Conventions.

Some children had to take part in military-patriotic training, including singing the Russian anthem, and to follow the Russian school curriculum - in violation of international humanitarian law.

The transfer of at least 200 children within Russian occupied territory and to Russia between February 2022 and December 2024 may amount to war crimes, the report stated.

Previously Moscow said it had been protecting vulnerable children from a war zone.

Ukraine has called the abductions of tens of thousands of its children taken to Russia or Russian-occupied territory without the consent of family or guardians a war crime that meets the U.N. treaty definition of genocide.