Ukraine has condemned a decree signed by Russian President Vladimir Putin making it possible to confer Russian citizenship on Ukrainian children moved to Russia.
Last March, the International Criminal Court issued an arrest warrant for Putin over Russia's policy of forced child deportations.
The Ukrainian foreign ministry called the decree illegal.
However, Russia insists it is moving the children out of harm's way.
On 4 January, Putin signed a citizenship decree expediting Russian citizenship to foreigners and stateless people.
The Ukrainian foreign ministry singled out the passage saying that orphaned Ukrainian children or those deprived of parental guardianship can be fast-tracked to Russian citizenship by way of a presidential decision, or after a request by the institution holding them.
The decree states that a citizenship application for such a child can be submitted by their legal guardian or the head of a Russian organisation responsible for the child.
The Ukrainian foreign ministry sees this as Russia's attempt to solve its own demographic crisis, describing it as a violation of Ukrainian and international laws and children's rights.