Ukraine said on Friday Russia had forcibly deported more than 210,000 children since its invasion on Feb. 24 and accused Moscow of wanting to make them Russian citizens.
Human rights ombudswoman Lyudmyla Denisova said the children were among 1.2 million Ukrainians who Kyiv says have been deported against their will.
Reuters could not independently verify the figure given by Denisova or her allegations, for which she did not provide supporting evidence.
The Kremlin did not immediately respond to a Reuters request for comment on Denisova's allegations concerning the deportation of large numbers of children and other Ukrainian nationals.
Moscow has denied intentionally targeting civilians since launching what it calls a special military operation in Ukraine and says it is offering humanitarian aid to those who want to leave Russia.
"When our children are taken out, they destroy the national identity, deprive our country of the future," Denisova said on national television.
"They teach our children there, in Russian, the history that (Russian President Vladimir) Putin has told everyone."
Russia has referred to "refugees" coming to Russia to escape fighting, particularly from the southern Ukrainian city of Mariupol, which is in Russian hands after weeks of siege and bombardment.
The 1949 Geneva Conventions, which define international legal standards for humanitarian treatment in conflict, prohibit mass forcible transfers of civilians during a conflict to the territory of the occupying power, classifying it as a war crime.
(Reporting by Natalia Zinets, Editing by Timothy Heritage and Jon Boyle)