The report said Yale University researchers had identified at least 43 camps and other facilities where Ukrainian children have been held that were part of a "large-scale systematic network" operated by Moscow since its February 2022 invasion of Ukraine.
The children included those with parents or clear familial guardianship, those Russia deemed orphans, others who were in the care of Ukrainian state institutions before the invasion and those whose custody was unclear or uncertain due to the war, it said.
Some of the children were moved through the system and adopted by Russian families, or moved into foster care in Russia, the report said.
The youngest child identified in the Russian program was just four months old, and some camps were giving military training to children as young as 14, Nathaniel Raymond, one of the researchers, said, adding that researchers had not found evidence those children were later deployed in combat.
Russia's embassy in Washington did not immediately respond to a request for comment on the report.
Moscow has denied intentionally targeting civilians in what it calls a "special military operation" in Ukraine and has pushed back against previous claims it had forcibly moved Ukrainians.
The report was the latest produced by the Yale University School of Public Health's Humanitarian Research Lab as part of a State Department-backed project that has examined human rights violations and war crimes allegedly committed by Russia.
Ukrainian prosecutors have said they are examining allegations of forced deportation of children as part of efforts to build a genocide indictment against Russia.
Nato allies weigh more arms for Ukraine as Russian artillery batters Bakhmut.
Reuters