The United Nations children's agency UNICEF says there has been a 300% increase in the number of unaccompanied and separated children registered in Greece.

It has welcomed a new Greek law, which came into force on April 4th, exempting certain vulnerable groups - including unaccompanied and separated children, children with disabilities and women who recently gave birth - from 'exceptional border procedures' or returns.

But it says more needs to be done.

Currently, more than 22,000 refugee and migrant children are stranded in Greece. since an agreement between the European Union and Turkey went into effect last month.

UNICEF is calling for a well-managed process to be put in place to determine each child's best interests.

"Any decision about any child, whether a toddler or a teenager, whether with family or not, should be guided by the best interests of that child," executive director of UNICEF Ireland Peter Power said.

"Children need to be heard. A rushed decision to return can lead to a rash result and going back to a place of fear and violence".

"Children, no matter where they come from, must have access to basic services at all times," he added.

UNICEF says it is concerned about reports that some children are being detained due to their migration status.

It adds that unaccompanied and separated children are among the most vulnerable - and make up about 10% of all refugee and migrant children in Greece.

But it says that not all are registered.

Figures show that between January and mid-March, 1,156 unaccompanied and separated children had been registered in Greece - an increase of over 300% compared to the same period in 2015.