WASHINGTON: Russia has held at least 6,000 Ukrainian children - likely many more - in sites in Russian-held Crimea and Russia whose primary purpose appears to be political re-education, according to a U.S.-backed report published on Tuesday.

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.

"The primary purpose of the camp facilities we've identified appears to be political re-education," Nathaniel Raymond, one of the researchers, said in a briefing to reporters.

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, Raymond 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.

"What is documented in this report is a clear violation of the 4th Geneva Convention," the agreement that protects civilians in wartime, said Raymond. He said it could also be evidence that Russia has committed genocide during its war in Ukraine, since the transfer of children for purposes of changing, altering or eliminating national identity can constitute a component act of the crime of genocide.

Ukrainian prosecutors have said they are examining allegations of forced deportation of children as part of efforts to build a genocide indictment against Russia.

"This network stretches from one end of Russia to the other," Raymond said, adding that researchers believed that the number of facilities in which Ukrainian children have been held exceeds 43.

The system of camps and the adoption by Russian families of Ukrainian children taken from their homeland "appears to be authorized and coordinated at the highest levels of Russia's government," the report said, beginning with President Vladimir Putin and extending to local officials.

State Department spokesperson Ned Price indicated that action could be taken against 12 individuals the report said are not yet under U.S. sanctions.

"We are always looking at individuals who may be responsible for war crimes, for atrocities inside of Ukraine," he said. "Just because we have not sanctioned an individual to date says nothing about any future action that we may take."