Russia is violating the “fundamental principles of child protection” in wartime by giving Ukrainian children Russian passports and putting them up for adoption, the U.N.’s refugee agency (UNHCR) chief told Reuters in an interview.

Speaking at the UNHCR offices in Kyiv following a six-day tour of Ukraine, Filippo Grandi said President Volodymyr Zelenskiy had asked his agency to “do more” to help children from occupied regions to whom this was happening.

“Giving them (Russian) nationality or having them adopted goes against the fundamental principles of child protection in situations of war,” Grandi said.

“This is something that is happening in Russia and must not happen,” he added.

Zelenskiy, after meeting Grandi on Wednesday, called for mechanisms to be established to “defend and return” children and adults deported to Russia since its invasion of Ukraine last February, as well as to punish those responsible.

Grandi said his agency was unable to estimate the number of children who had been given passports or put up for adoption, as access in Russia was extremely limited.

“We are seeking access all the time, and access has been rather rare, sporadic and not unfettered,” he said.

In Moscow, foreign ministry spokeswoman Maria Zakharova accused Grandi of being silent when children died as a result of what she said was Ukrainian shelling in the Donbas region after pro-Moscow separatists declared independence in 2014.