US Secretary of State Antony Blinken has called on Russia to stop forced deportations and "filtration" operations involving Ukrainians.
Russia had detained and forcibly moved to Russia some 900,000 to 1.6 million people, many of them children, he said.
Mr Blinken cited reports of children being separated from parents or abducted from orphanages to be put up for adoption in Russia.
There was also mounting evidence of torture or summary execution, he said.
Russia has in the past rejected reports of filtration camps as unprofessional and lies.
Mr Blinken, who was speaking on the eve of a conference in The Hague on alleged war crimes in Ukraine, said unlawfully transferring or deporting civilians was a grave breach of the Fourth Geneva Convention.