Moscow will be asked to explain at the UN on Monday what has happened to thousands of Ukrainian children believed to have been forcibly sent to Russia since its 2022 invasion.
Kyiv estimates that 20,000 Ukrainian children have been forcibly deported to Russia.
The International Criminal Court (ICC) issued an arrest warrant against Russian President Vladimir Putin in March 2023 on the war crime accusation of unlawfully deporting Ukrainian children.
The United Nations Committee on the Rights of the Child -- a panel of 18 independent experts -- is set over two days to examine Russia's record, as part of a regular review.
They want to know how many children have been "evacuated" to Russia or within Russian-occupied parts of Ukraine.
They also want to know what Moscow has done to protect "the right of such children to preserve their identity, including nationality, name and family relations".
Their lengthy list of concerns was sent to Moscow in the first half of 2023.
- Return mechanism idea -
Russia launched its full-scale invasion of Ukraine in February 2022 and says it wants to protect these children from the fighting.
Only around 400 children have so far been repatriated.
Such placements are arranged at the children's "request and with their consent," Russia said in a written response sent in October and shown to media by the UN on Friday.
It does not specify the total number of children affected, but said they "included children from national residential institutions for orphans and children without parental care (about 2,000 in total)" and children with Ukrainian citizenship.
It also said that in the second quarter of 2023, some 46,886 Ukrainian children acquired Russian citizenship.
Kateryna Rashevska, a legal expert with the Regional Center for Human Rights, a Ukrainian NGO, hopes the UN committee will call for an "international legal mechanism" to identify and return the children.
"The international community is ready to do something but there is a need to do it faster," she told AFP.
At the current rate, "we need another 90 years in order to repatriate only already-identified Ukrainian children".
The ICC has also levelled similar charges to Putin's against Maria Lvova-Belova, Russia's presidential commissioner for children's rights.
Russia, which is not a member of the ICC, insists the warrant against Putin is "void".
- Broader crackdown context -
The UN will also seek answers on what Moscow is doing to remove obstacles to children exercising their right to freedom of association and assembly, and to ensure that children are not punished for taking part in demonstrations, particularly against the war in Ukraine.
Russian lawyer Olga Sadovskaya, who heads the Crew Against Torture human rights organisation, said Russia had seen an increase in violence "in every part of life", which therefore affects children.
She noted a sharp increase in domestic violence and the severity of brutality when people are arrested.
In a report sent to the UN committee, the NGO Human Rights Watch said it was concerned about children's freedom of expression, their right to freedom of information, and discrimination linked to gender identity and sexual orientation.
Rachel Denber, deputy director of HRW's Europe and Central Asia division, told AFP this demonstrated how the "broader crackdown on rights in Russia has an impact on children's rights as well", noting how youngsters had faced "retaliation" for voicing critical opinions on Russia's war in Ukraine.
The UN committee also wants to discuss the illegal or arbitrary detention of children, corporal punishment, and measures taken to preserve the cultural and linguistic identity of indigenous children.
The experts want to know what Moscow is doing to combat certain "harmful practices" in the North Caucasus, such as child marriage, female genital mutilation, abductions for forced marriages and polygamy.