Ukraine President Volodymyr Zelensky urged countries to stand firm against Russia's "genocide," including the forcible deportation of 19,000 Ukrainian children to Russia, in his address to the United Nations General Assembly in New York on Tuesday.

"This is a real chance for every nation — to ensure that aggression against your state, if it happens, God forbid, will end not because your land will be divided," Zelensky said, but with sovereignty upheld.

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

In his own address, United States President Joe Biden expressed support for Ukraine.

"We must stand up to this naked aggression today to deter other would-be aggressors tomorrow," Biden said.

The US leader said allowing Ukraine to be carved up threatens the security of any nation.

Speaking on the sidelines of the UNGA, Ukraine's First Lady, Olena Zelenska, said that Ukrainian children taken to Russia "were told that their parents don't need them, that their country doesn't need them, that nobody is waiting for them."

"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," she pleaded.

Only 386 Ukrainian children have been brought back and more than 500 have been killed since Russia invaded Ukraine, according to Zelenska.

Russia denies the allegations, saying instead it has saved Ukrainian children from the horrors of the war that started in February 2022 when Moscow launch its special military operations in its neighbor.

The International Criminal Court has issued an arrest warrant for Russian President Vladimir Putin for the deportation considered as a crime against humanity. Another warrant was issued for Maria Lvova-Belova, Russia's presidential commissioner for children's rights, on similar charges.