Ukraine's first lady urged world leaders on Tuesday to help return Ukrainian children forcibly taken to Russia, where she said they are being indoctrinated and deprived of their national identity.

Speaking on the sidelines of the United Nations General Assembly, Olena Zelenska said that more than 19,000 Ukrainian children have been transferred by force or deported to Russia or occupied territories.

So far, only 386 have been brought back.

In Russia, "they were told that their parents don't need them, that their country doesn't need them, that nobody is waiting for them," Zelenska said.

"The abducted children were told that they are no longer Ukrainian children, that they are Russian children."

In his address to the General Assembly earlier in the day, President Volodymyr Zelensky went further, calling Russia's actions genocidal.

"We are trying to get children back home but time goes by. What will happen to them?" he asked.

"Those children in Russia are taught to hate Ukraine, and all ties with their families are broken," Zelensky added. "This is clearly a genocide."

Russia denies the allegations, saying instead it has saved Ukrainian children from the horrors of the war.

The International Criminal Court has issued an arrest warrant for Russian President Vladimir Putin on the war crime accusation of unlawfully deporting Ukrainian children. Another warrant was issued for Maria Lvova-Belova, Russia's presidential commissioner for children's rights, on similar charges.

- Sexual violence -

More than 500 children have been killed since Russia invaded its neighbor more than a year and a half ago, and hundreds of others have been maimed or wounded, according to Zelenska.

Ukrainian authorities are also investigating over 230 cases of sexual violence by Russian soldiers against civilians, including 13 children, according to the first lady. She said the child victims include 12 girls and one boy, with the youngest victim only four years old at the time of the crime. 

"I am turning to the UN secretary-general and the entire organization to help us save Ukrainian children," Zelenska pleaded.

"Help us receive information on the children taken to Russia... Help us take children out of occupied territories through special safe corridors. Our children need justice."

Ukrainian Prosecutor General Andriy Kostin said the international community, in order to avoid global food shortages, had successfully brokered a treaty with Russia allowing Ukrainian grain exports to continue despite the war.

He called for a similar legal mechanism to facilitate the return of Ukrainian children.

"The world was very active," Kostin said, referring to the adoption of the Black Sea Grain Initiative, which Russia has since exited. "I think now it's time for the United Nations to become united to return Ukrainian children back home."

He added: "Any voice, any pressure, any communication... would help."

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