At least 1500 unaccompanied migrant and refugee children stranded in Greece have nowhere safe to stay, many sleeping rough in the cold and others incarcerated, a charity warned ahead of Pope Francis' visit to Lesbos.

Save the Children said Greece had to stop locking up children and called on the European Union to help open more safe shelters.

"Children are sleeping rough in increasingly volatile unofficial accommodation sites, are being incarcerated in detention centres and are slipping through the cracks of the system," said Amy Frost, Save the Children's team leader in Greece. "They are at risk of violence, abuse and exploitation by people traffickers."

The children - some only 10 - come from countries including Syria, Afghanistan and Pakistan. Many have fled war and other violence. Some have travelled alone to Europe, while others have become separated from their families.

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Border closures have left an estimated 2000 unaccompanied children trapped in Greece, but Save the Children said there were just 477 shelter spaces for them in the country.

- AAP