Ukraine prosecutors investigating the war crimes cases in the country are examining allegations of the forcible deportation of children to Russia since the invasion as they seek to build a genocide indictment.

International humanitarian law classifies the forced mass deportation of people during a conflict as a war crime. “Forcibly transfering children” in particular qualifies as genocide, the most serious of war crimes, under the 1948 Genocide Convention that outlawed the intent to destroy – in whole or in part – a national, ethnic, racial or religious group.

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In an interview with Reuters, prosecutor General Iryna Venediktova, who is overseeing multiple war crimes inquiries in Ukraine, said “we have more than 20 cases about forcible transfer of people” to Russia from various regions across the eastern European country since the invasion began on February 24.

“From the first days of the war, we started this case about genocide,” Venediktova told Reuters. She said that, amid the chaos and destruction wrought by Russia’s assault, focusing on the removal of children offered the best way to secure the evidence needed to meet the rigorous legal definition of genocide: “That’s why this forcible transfer of children is very important for us.”

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However, the prosecutor declined to provide a number for how many victims had been forcibly transferred. Ukraine’s human rights ombudswoman Lyudmyla Denisova said in mid May that Russia had relocated more than 210,000 children during the conflict, part of more than 1.2 million Ukrainians who Kyiv said have been deported against their will.

A Kremlin spokesman did not respond to a request for comment on Venediktova’s remarks nor the figures on Ukrainians on Russian soil. Russia in the past has said that it is offering humanitarian aid to those wishing to flee Ukraine voluntarily.

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Russia calls its actions in Ukraine a “special operation” to disarm Ukraine and protect it from fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.

The Genocide Convention – a treaty adopted by the U.N. General Assembly in the wake of the Nazi Holocaust – specifies five acts that could each constitute the crime, if committed with genocidal intent: killing members of a group, causing them serious bodily or mental harm, imposing living conditions intended to destroy the group, preventing births, and forcibly transferring children out of the group.

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Russia has strongly denied that its forces have committed war crimes in Ukraine and has in turn accused Ukrainian troops of atrocities, including mistreatment of prisoners of war. Kyiv has said allegations of abuse will be investigated.