The US has called on Russia to immediately release Ukrainians it has forced out of their home country and allow the presence of outside observers, citing reports Moscow was putting Ukrainian children up for adoption and "disappearing" thousands of others.
"The unlawful transfer and deportation of protected persons is a grave breach of the Fourth Geneva Convention on the protection of civilians and is a war crime," US Secretary of State Antony Blinken said in a statement before a conference scheduled on Thursday to address "accountability" in the conflict.
Mr Blinken said reports indicated Moscow was "deliberately separating Ukrainian children from their parents and abducting others from orphanages before putting them up for adoption inside Russia" while also "detaining or disappearing thousands of Ukrainian civilians who do not pass 'filtration'".
Ukrainian officials have accused Russia of deporting hundreds of thousands of people from Russian-occupied areas of Ukraine and forcing them into so-called filtration camps.
The Russian embassy in the US said the comments made by Washington were an attempt to stoke "Russophobia".
"Washington's attempt to vilify the Armed Forces of the Russian Federation is apparently connected with dissatisfaction with the success of a special military operation," Russia's embassy said in a Facebook post.
The embassy also said that the humanitarian corridors were also operational in the region and added that "statements about the forcible confiscation of Ukrainian passports are poor-quality western disinformation".
The 1949 Geneva Conventions, which define international legal standards for humanitarian treatment in conflict, prohibit mass forcible transfers of civilians to the territory of the occupying power, classifying it as a war crime.
"Estimates from a variety of sources, including the Russian government, indicate that Russian authorities have interrogated, detained and forcibly deported between 900,000 and 1.6 million Ukrainian citizens, including 260,000 children, from their homes to Russia – often to isolated regions in the Far East," Mr Blinken said.
Moscow, which invaded Ukraine on February 24, has denied intentionally attacking or displacing civilians.