Six Ukrainian children will be returned to Ukraine from Russia under a deal brokered by Qatar, a Qatari official said on Tuesday, with a source involved in organising the returns saying they had been staying with relatives in Russia or Russian-occupied territory.

The children are en route to Ukraine via Moscow, the source added.

This is the second phase of a Qatar-mediated return of children, after four minors were returned in October.

Negotiations on the returns had been underway since at least April 2023, a source told Reuters in July.

Qatar agreed to a Ukrainian request to mediate with Russia on the return of children to their immediate families during a visit to Ukraine in July 2023 by Qatari Prime Minister Sheikh Mohammed bin Abdulrahman al-Thani.

"The process involved attaining the consent of the families, ... identification of minors and verification of identification information, coordination with humanitarian organizations, as well as logistical arrangements," the Qatari official said.

One of the children was being escorted by Qatari diplomats to their embassy in Moscow before being transferred to Ukraine, the source said.

The office of Ukraine's human rights ombudsman was not immediately available for comment.

The cases appear to be different to those of Ukrainian children who Ukraine says were forcibly taken to Russia from territories occupied by Moscow, and which are the subject of an International Criminal Court case.

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 the accusation and says it has protected vulnerable children from the war zone.

The case has led to Russian President Vladimir Putin and Children's commissioner Maria Lvova-Belova being issued with ICC arrest warrants.

The ICC has accused them of responsibility for the war crime of deporting Ukrainian children - at least hundreds, possibly more - to Russia.

The Kremlin has rejected the charges, saying they have "no meaning" in Russia. The ICC's 123 member states are obliged to detain and transfer Putin if he sets foot on their territory. Russia is not a member. (Reporting by Andrew Mills in Doha and Max Hunder in Kyiv, Editing by Alexandra Hudson)