President Volodymyr Zelensky said on Friday he had asked his Chinese counterpart Xi Jinping to help bring back Ukrainian children deported by Russia.

"We need to involve everyone... to put pressure on the Russian aggressor and the terrorists who kidnapped so many of our children," Zelensky said.

"The UN, many others want to do something, but so far the results have been poor. So I have appealed to the leader of China," he said.

Xi and Zelensky spoke by phone on Wednesday, the first known call between the two leaders since the start of Russia's military campaign.

Beijing says it is neutral in the Ukraine conflict and Xi has never condemned the Russian invasion.

Xi presented himself as a mediator concerned with maintaining stability when he visited Russian President Vladimir Putin in March.

Putin is under an arrest warrant from the International Criminal Court over the "unlawful deportation" of Ukrainian children.

More than 16,000 Ukrainian children have been deported to Russia since the February 24, 2022, invasion, according to Kyiv.

It says many of them have been placed in institutions and foster homes, an allegation denied by Russia, which insists it saved Ukrainian children from the horrors of the war. (AFP)