Kyiv accused Moscow on Tuesday of having organized illegal mass adoptions of Ukrainian children after transferring them from occupied territories to Russia.
Since the beginning of the war, Kyiv has been accusing Moscow of "deporting" Ukrainians, saying Ukrainians from occupied territories have been forced to go to Russia rather than other regions of Ukraine.
"The Russian Federation continues to abduct children from the territory of Ukraine and arrange their illegal adoption by Russian citizens," Ukraine's foreign ministry said in a statement.
"Over 1,000 children from Mariupol," a southern Ukrainian city occupied by Russian troops, "were illegally transferred to outsiders in Tyumen, Irkutsk, Kemerovo, and Altai Krai" (in Siberia), the statement read, referring to different areas of Russia.
The foreign ministry said it had based its findings on information from local authorities in Krasnodar, a southern Russian city near Ukraine.
More than 300 Ukrainian children are "held in specialized institutions" in the Krasnodar region, according to the statement.
The ministry accused Russia of actions that "grossly violate the 1949 Geneva Convention" that establishes rules for humanitarian treatments in wartime and the UN Convention on the Rights of the Child.
It called for "all Ukrainian children, who were illegally displaced to the territory of Russia, (to) be returned to their parents or legal guardians."
Several families from Mariupol told AFP that they had been forced to go to Russia to flee the fighting.
Mariupol, a strategic port city on the Sea of Azov, was surrounded in the early days of the invasion.
Russia fully seized the city after weeks of siege and intense shelling that left around 20,000 dead, according to Ukrainian estimates.