Ukrainian officials say more than 121,000 children have been forcibly deported to Russia.
Russia is changing legislation to allow for the fast-tracked adoption of these children, they allege.
The Ukrainian Presidential Advisor on Children's Rights accused Russia of violating UN laws.
The Ukrainian Ombudswoman for Human Rights has said that the Russian government is crafting legislation to allow Russians to adopt Ukrainian children forcibly taken to Russia by military forces.
She added that Ukrainian officials have no information on the children that are allegedly being processed for adoption by Russians.
Also writing on Facebook, the Ukrainian Presidential Advisor on Children's Rights Herasymchuk Daria said that "Such adoption is a violation of the child's rights.
On April 8, The UN stated that of the 1626 Ukrainians officially killed so far, 63 were children. »