As Russian forces laid siege to the Ukrainian city of Mariupol this spring, children fled bombed-out group homes and boarding schools. Separated from their families, they followed neighbors or strangers heading west, seeking the relative safety of central Ukraine.

Instead, at checkpoints around the city, pro-Russia forces intercepted them, according to interviews with the children, witnesses and family members. The authorities put them on buses headed deeper into Russian-held territory.

“I didn’t want to go,” said Anya, 14, who escaped a home for tuberculosis patients in Mariupol and is now with a foster family near Moscow. “But nobody asked me.”

In the rush to flee, she said, she left behind a sketchbook containing her mother’s phone number. All she could remember were the first three digits.

Since Russia’s invasion of Ukraine began in February, Russian authorities have announced with patriotic fanfare the transfer of thousands of Ukrainian children to Russia to be adopted and become citizens. On state-run television, officials offer teddy bears to new arrivals, who are portrayed as abandoned children being rescued from war.

In fact, this mass transfer of children is a potential war crime, regardless of whether they were orphans. And while many of the children did come from orphanages and group homes, the authorities also took children whose relatives or guardians want them back, according to interviews with children and families on both sides of the border.

As Russian troops pushed into Ukraine, children like Anya who were fleeing newly occupied territories were swept up. Some were taken after their parents had been killed or imprisoned by Russian troops, according to local Ukrainian officials.

This systematic resettlement is part of a broader strategy by the Russian president, Vladimir V. Putin, to treat Ukraine as a part of Russia and cast his illegal invasion as a noble cause. His government has used children — including the sick, poor and orphaned — as part of a propaganda campaign presenting Russia as a charitable savior.