While illegal migration to Greece has fluctuated, numbers are nowhere near 2015 figures, according to the International Organization for Migration. Smugglers adapted to heightened surveillance, shifting to more dangerous routes.

Last year, EU nations approved a migration and asylum pact laying out common rules for the bloc's 27 countries on screening, asylum, detention and deportation of people trying to enter without authorization, among other things.

The deal, endorsed by the United Nations High Commissioner for Refugees, takes effect next year. Critics say it made concessions to hard-liners. Human rights organizations say it will increase detention and erode the right to seek asylum.