Eleven Ukrainian children crossed the border from Belarus to Ukraine Tuesday evening, in the latest return of children taken to Russia and occupied territories during the nearly two-year Ukraine war.

Emerging from the darkness at a humanitarian crossing on the Belarus border, the children hugged family members who had been waiting for more than six hours.

Oleksandr, 16, is the oldest among those returned by Moscow through a Qatar-mediated scheme.

"My new life is starting," he said, smiling shyly and describing the "joy and slight nerves".

The children were received by the Qatari embassy in Moscow on Monday before travelling to Belarus and walking across the one-kilometre border zone -- while some relatives were able to meet the children directly in Moscow.

Two critically ill children were brought over in an ambulance and rushed to hospital.

Ukraine estimates 20,000 children have been forced to Russia since the war erupted in February 2022.

President Volodymyr Zelensky has called the action "a genocide". Russia denies the accusations.

The group of children is the fourth and largest to have been returned with Qatar's help and included some as young as two, Ukrainian Human Rights Commissioner Dmytro Lubinets told AFP at the border.

"Believe me, we will bring them all back," Lubinets assured the waiting relatives.

Oleksandr's aunt Viktoria, 47, had not seen him since the war broke out.

She unsuccessfully tried to collect her nephew three times and only managed to speak to him on the phone recently.

Officials in Russian-occupied Lugansk sent him to a state boarding school, similar to a children's home, where they took away his documents and "psychologically pressured him to stop him leaving", she said.

"Our situation seemed deadlocked."

Oleksandr was sent to the school after his mother and older brother, 21, were killed by shelling of their car as they tried to flee the Lugansk region in July 2022.

Sometimes Oleksandr dreams of his mother screaming as she died, his aunt added.

Now she plans to take her nephew to live with her in Zhytomyr near Kyiv.

"We will celebrate and show him the city."

Computer developer Sergiy, 36, from Kyiv, also pulled his niece and nephew into a tight embrace as he collected them at the border.

After their parents died, Lev, 13, and Zhazmin, 10, lived with a distant relative in their home city of Russian-occupied Mariupol.

The relative moved them to the suburbs of Moscow as Mariupol became a fierce battlefield in the spring of 2022, before later returning to the Ukrainian city.

The relative "had no desire to take care of the children" so she tried to put them in a state children's home, Sergiy said.

"I thought it was almost impossible to get the kids back."

Smiling, Sergiy said he was ready to become a father of two, having no children himself.

"I will try to show them what it is like when they are needed and when someone can properly care of and support them."

Another mother, who wished to remain anonymous, collected her 13-year-old son after she was held prisoner in Mariupol.

"With an intermediary... we have new approaches, and you can see the result," Lubinets said.

Lubinets added that he had just returned from meeting Qatar's prime minister to discuss the return of both children and civilians.

"I can't disclose the details publicly yet, but I will say that I saw the maximum interest for Qatar to take part in this."

For his part, Qatari ambassador Hadi Nasser Mansour Al-Hajri told AFP that the country was ready to help bring out more people.

"If there is a request from both sides, we will do it, we are eager to do it."

"We are open for any possibilities: bringing prisoners of war or political prisoners... and the kids, we are open for all these things."

Since July 2023, Qatar has helped bring out almost 30 children, the ambassador said.

"We are almost the only country involved in the issue so we will continue."