By Thomas Peter and Yurii Kovalenko

KORTELISY, Ukraine (Reuters) -A Ukrainian teenager who was taken to Russia from the occupied city of Mariupol during the war and prevented from leaving the country earlier this year returned to Ukraine on Sunday.

Bohdan Yermokhin, who turned 18 on Sunday, appealed to President Volodymyr Zelenskiy this month to help bring him back to Ukraine. In March, he tried to leave Russia for Ukraine via Belarus, but was stopped and sent back.

"I believed I would be in Ukraine, but not on this day," Yermokhin told Reuters while eating at a petrol station after crossing into Ukraine.

"This is a very pleasant gift, to put it in the right way. The emotions are overwhelming, all good, with the notion that Ukraine needs me."

Zelenskiy welcomed Yermokhin's return in his nightly video address.

"Many attempts were made to help him. I am happy everything worked out," he said, expressing thanks to Ukrainian officials, international organisations, and particularly the U.N. Children's Fund, UNICEF, and authorities in Qatar for help in mediation.

Ukraine says 20,000 children have been illegally transferred to Russia since the invasion on Feb. 24, 2022, with some being put up for adoption. Kyiv says this is a war crime, an allegation denied by Russia, which says it was protecting children in a war zone.

ORPHAN PLACED IN FOSTER CARE NEAR MOSCOW

Yermokhin, an orphan from the Ukrainian city of Mariupol that was captured by Moscow's troops during the first year of the war, was taken to Russia and placed in a foster family in the Moscow region.

On Sunday, Reuters correspondents at Kortelisy, a Ukrainian village near the border with Belarus, saw Yermokhin driven into Ukraine from the border in a van. Asked if he was glad to be back in Ukraine, Yermokhin said "yes."

"We were in constant contact with Bohdan and he's already in Ukraine with his cousin," Andriy Yermak, head of the president's office, wrote on Telegram messenger, announcing his return.

Mariam Lambert of the Dutch NGO Orphans Feeding Foundation told Reuters they have been working with Ukraine's human rights ombudsman and Zelenskiy's office on the return of children deported to Russia, including Yermokhin, since August.

His lawyer, Kateryna Bobrovska, has said Yermokhin had been told to report to a draft office near Moscow next month and warned he could be conscripted into the Russian army.

In a statement, Russia's children's commissioner, Maria Lvova-Belova, said he had been summoned to update his military registration and that "all Russian citizens of his age receive a summons of that kind."

Lvova-Belova said Yermokhin left Russia on Saturday on a plane to Minsk on his way to Ukraine and that he had met a cousin in the Belarusian capital. She acknowledged Yermokhin had wanted to be reunited with his relative.

The International Criminal Court (ICC) in The Hague issued an arrest warrant for Russian President Vladimir Putin in March, accusing him and Lvova-Belova of the war crime of illegally deporting hundreds of children from Ukraine.

The Kremlin says Moscow does not recognise the jurisdiction of the ICC and has rejected the allegations.

(Additional reporting by Pavel Polityuk; Writing by Tom Balmforth; Editing by Jane Merriman, Ron Popeski, Bill Berkrot and Chris Reese)