A 7-year-old Ukrainian boy is seated next to his grandmother (left) and Russia's Commissioner for Children's Rights, Maria Lvova-Belova after being released to Qatari diplomats at Qatar's embassy in Moscow, Russia. File / Reuters

Qatar is helping four Ukrainian children return home from Russia this week under a mechanism Doha has set up with cooperation from both sides, and hopes to help bring more home in the future, a Qatari official told Reuters on Monday.

A seven-year-old boy returned to Ukraine on Monday via Estonia after he was reunited with his grandmother in Qatar's Moscow embassy last week, the official said.

A two-year-old boy was handed over to Qatari diplomats in Moscow on Monday, and a nine-year-old boy and a 17-year-old girl are due to be handed over this week, the official said.

The Ukrainian Presidential Office said the children would all be home soon.

"We are bringing home four Ukrainian children illegally deported to the Russian Federation," Andriy Yermak, head of the presidential office, said on X, formerly known as Twitter. He gave no further details.

Kyiv says about 20,000 children have been taken from Ukraine to Russia or Russian-held territory without the consent of family or guardians. It calls this a war crime that meets the U.N. treaty definition of genocide.

Moscow, which launched a full-scale invasion of Ukraine in February last year, denies committing the accusation and says it has protected vulnerable children from the war zone.

The return of the first four children tests a system the Gulf Arab state established after months of secret talks with Moscow and Kyiv, the Qatari official said on condition of anonymity because of the sensitivity of the process.

Qatar's minister of state for international cooperation, Lolwah Al Khater, confirmed the mediation in a statement and described this week's repatriations as "only a first step".

Russia's Commissioner for Children's Rights, Maria Lvova-Belova, shared a short initial list of Ukrainian children to be returned with a team of Qatari diplomats who verified each child's identity, the official said.

It was not clear how many additional children Russia would authorise to return to Ukraine via the Qatari mechanism, the Qatari official said.

About 400 children have been returned to Ukraine since Moscow's invasion, but the United Nations human rights agency raised concerns last week that there was no system to facilitate such returns.

Qatari diplomats will accompany the children over the border with Estonia, Latvia or Belarus or to Qatar by chartered jet before returning them to Ukraine, the Qatari official said.

"Both Ukrainian and Russian officials have been cooperative," the Qatari official said, adding that Ukraine approached Doha to mediate with Russia over the children.

Qatar has recently brokered a US-Iran prisoner swap and mediated between Israel and Hamas militants over the potential release of Israeli hostages held in Gaza.

The seven-year-old boy reunited with his grandmother on Friday had been in Russia at a children's home "as a result of being separated from his mother who was in Russia when the war broke out", the Qatari official said.

The International Criminal Court issued arrest warrants in March for Russian President Vladimir Putin and Lvova-Belova, accusing them of the war crime of illegally deporting children from Ukraine.

Lvova-Belova, who denies the war crime allegations against her, told reporters in Moscow that the indictments were based on "a massive campaign of disinformation (that) has been launched by the Kyiv regime".

She said the Ukrainian list of nearly 20,000 children in Russia included some who had already been reunited with their parents.

Reuter