(CNN) Child migrants are being abused, detained and returned to Italy illegally by French police, according to a report published Friday by the charity Oxfam.

Some children trying to cross the border near the Italian coastal town of Ventimiglia have had their mobile phones seized and SIM cards stolen by the authorities, while others have been sent back to Italy after having had the soles of their shoes cut off, according to the report.

"French police officers are not upholding international standards," said Chiara Romagno from Oxfam Italy. "It's unacceptable. Besides pushing them back illegally, without offering any guarantees, against the law, they taunt them and mistreat them."

The French interior ministry did not respond to a request for comment from CNN.

Earlier this year, an administrative tribunal in Nice ruled that the police violated legislation in 20 separate cases of unaccompanied children who were detained and returned across the border to Italy.

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