A United Nations envoy for human rights has criticized the European Union's response to the refugee crisis as showing a "lack of vision," operating under legal ambiguity, and backing the detention of newly arrived migrants in Greece.

Envoy Francois Crepeau said the March agreement between the EU and Turkey to send back migrants reaching Europe required "much stronger legal instruments to ensure legal accountability."

Greece has been detaining migrants and refugees who arrived after March 20, while some 50,000 people are stranded in the country since European countries, including Austria and Balkan states, closed their borders to migrants earlier this year.

Crepeau expressed concern that children were among those detained, and urged an immediate change in policy.

He spoke in Athens Monday after visiting detention sites and migrant camps.