Prosecutors investigating war crimes cases in Ukraine are looking into charges of forceful repatriation of children to Russia since the invasion, according to the country’s chief prosecutor in an interview.

Forced mass expulsion of civilians during a conflict is classified as a war crime under international humanitarian law. “Forcibly transferring children” in particular qualifies as genocide, the most heinous of war crimes, under the 1948 Genocide Convention, which prohibited the purpose to eliminate a national, ethnic, racial, or religious group in whole or in part.

Since the invasion began on Feb. 24, Ukraine’s Prosecutor General Iryna Venediktova has declared that “we have more than 20 complaints involving forcible transfer of persons” to Russia from various districts throughout the eastern European country.

“We launched this argument regarding genocide from the beginning of the conflict,” Venediktova told Reuters. She said that, in the midst of the turmoil and devastation caused by Russia’s invasion, focusing on the evacuation of children was the only approach to obtain the evidence required to fulfill the strict legal definition of genocide: “That is why the forceful transfer of children is so critical to us.”

Venediktova refused to say how many victims had been forcefully moved. However, Ukraine’s human rights ombudswoman, Lyudmyla Denisova, stated in mid-May that Russia had transferred more than 210,000 children during the conflict, as part of the more than 1.2 million Ukrainians deported without their will, according to Kyiv.