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