The UN Commission of Inquiry on Ukraine said there was evidence of hundreds of Ukrainian children being illegally transferred to Russia.
The commission’s report also includes information that Russia has committed other war crimes in Ukraine.
These include attacks on hospitals, torture, rape and deliberate killings.
Ukrainian government figures put the number of children forcibly taken to Russia at 16,221.
The report states that Russia has initiated policies such as granting Russian citizenship and placing children in foster families “to create a framework in which some children can stay in Russia permanently”.
UN investigators note that while transfers should be temporary, they are protracted as both parents and children face “a range of barriers to contact”.
The report states that the forced deportation of Ukrainian children “violates international humanitarian law and constitutes a war crime”.