Monitors from Turkey have arrived on the Greek islands of Lesbos and Chios to help supervise an agreement aimed at limiting the number of refugees and migrants travelling to the European Union.
The monitors arrived on Monday as Greek authorities scrambled to implement the landmark deal reached last week between the leaders of the EU and Turkey that includes faster refugee relocations to European countries as well as collective deportations from Greek islands to Turkey.
Government figures released today said the number of stranded migrants in Greece exceeded 50,000, with no significant changes in the number of daily arrivals.
Under the terms of the deal, migrants arriving on Greek islands from Sunday onward will not be allowed to travel to the Greek mainland.
Greece's conservative opposition criticised the Turkish arrivals, a controversial topic as Greece and Turkey have ongoing boundary disputes in the Aegean Sea.
"I think it is particularly insulting to have Turkish officers operating on Greek territory," Miltiadis Varvitsiotis, a former merchant marine minister, told private Skai television.
Four migrants died on Sunday while trying to reach Greece, two men off the island of Lesbos and two girls off the tiny islet of Ro, the coast guard said, as smugglers appeared to be opting for more overnight trips and increasingly dangerous routes.
Panos Skourletis, Greece's environment minister, said smugglers needed to be stopped for the deal to work.
"On paper, the agreement is balanced and the best result we could get," he told private Mega television.
"But no one can guarantee that it will work in practice. That will become clear over the next few days."
Under the deal, migrants arriving on Greek islands from Sunday onward will not be allowed to travel to the Greek mainland.
But authorities here are still working out how newly-arrived refugees and migrants will be sent back to Turkey.
EU migration commissioner Dimitris Avramopoulos and Prime Minister Alexis Tsipras were meeting on the topic Monday in Athens.
On the Greek mainland, army personnel continued expanding refugee shelters at sites in central and northern Greece - mostly at former army bases - so migrants who travelled to the Greek islands before the agreement came into effect could be resettled.
Ferries transporting them from the islands were using the port of Elefsina, west of Athens, instead of the main commercial port of Piraeus, to allow authorities to cope with the increased numbers.