More than 30 children were reunited with their families in Ukraine this weekend after a long operation to bring them back home from Russia or Russian-occupied Crimea, where they had been taken from areas occupied by Russian forces during the war.

Mothers hugged sons and daughters as they crossed the border from Belarus into Ukraine on Friday after a complex rescue mission involving travel across four countries.

Dasha Rakk, a 13-year-old girl, said she and her twin sister had agreed to leave the Russian-occupied city of Kherson last year because of the war and go to a holiday camp in Crimea for a few weeks. But once in Crimea, Russian officials said the children would be staying for longer.

"They said we will be adopted, that we will get guardians," she said. "When they first told us we will stay longer we all started crying."

Dasha's mother Natalia said she had travelled from Ukraine to Crimea via Poland, Belarus and Moscow to get her daughters. Ukraine's Crimea peninsula has been occupied by Russia since 2014.

"It was terribly difficult but we kept on going, we did not sleep at nights, we slept sitting up," she said, describing her journey to the camp.

"It was heartbreaking to look at children left behind who were crying behind the fence," she said.

Kyiv estimates nearly 19,500 children have been taken to Russia or Russian-occupied Crimea since Moscow invaded in February last year, in what it condemns as illegal deportations.

Moscow, which control chunks of Ukraine's east and south, denies abducting children and says they have been transported away for their own safety.

"Now the fifth rescue mission is nearing its completion. It was special regarding the number of children we managed to return and also because of its complexity," said Mykola Kuleba, the founder of the Save Ukraine humanitarian organisation that helped arrange the rescue mission.

Kuleba told a Kyiv briefing on Saturday that all 31 children brought home said no one in Russia was trying to find their parents.

"There were kids who changed their locations five times in five months, some children say that they were living with rats and cockroaches," he said. The children were taken to what Russians called stays in summer camps from occupied parts of Ukraine's Kharkiv and Kherson regions, Kuleba said.

The Russian Foreign Ministry did not immediately respond to a request for comment.

Arrest Warrants

Three children - two boys and a girl - were present at the media briefing in Kyiv. Save Ukraine said they came home on a previous mission last month that returned 18 children in total.

The three said they had been separated from their parents who were pressured by Russian authorities to send their children to Russian summer camps for what was billed as two weeks, from occupied parts of Kherson and Kharkiv regions.

The children at the briefing said they were forced to remain at the summer camps for four to six months and were moved from one place to another during their stay.

"We were treated like animals. We were closed in a separate building," said Vitaly, a child from Kherson region whose age was not clear. He added that they were told their parents no longer wanted them.

The International Criminal Court last month issued an arrest warrant for Russian President Vladimir Putin and Russia's children's rights commissioner, Maria Lvova-Belova, accusing them of abducting children from Ukraine.

Moscow has not concealed a programme under which it has taken thousands of Ukrainian children from occupied areas, but presents this it as a humanitarian campaign to protect orphans and children abandoned in the conflict zone.

Russia rejects the ICC allegations, saying it does not recognise the court's jurisdiction and calling the warrants null and void.

Lvova-Belova said earlier this week that her commission acted on humanitarian grounds to protect the interests of children in an area where military action was taking place and had not moved anyone against their will or that of their parents or legal guardians, whose consent was always sought unless they were missing.

Kateryna Rashevska, a lawyer from a Ukrainian NGO called Regional Centre for Human Rights, told the briefing they were collecting evidence to build a case that Russian officials deliberately prevented return of the Ukrainian children.

"In every story there is a whole range of international violations and it cannot go unpunished," she said.