Five Afghans, including two children and a six-month old baby, drowned in the Aegean overnight trying to reach Greece as Turkey starts to implement an EU agreement to curb illegal migration.
The stream of refugees risking perilous sea crossings has so far shown little sign of abating after Turkey reached the draft deal with the EU on Monday.
Turkey's EU Minister Volkan Bozkir said the deal, under which Ankara agreed to take back all irregular migrants, does not apply to those who have already reached the Greek islands and that the number returned was ultimately expected to be in the thousands or tens of thousands.
One of the victims from Wednesday night's capsize, in which their boat hit rough weather as it travelled towards the Greek island of Lesbos, managed to swim ashore but died during attempts to revive him, said a coast guard official.
Of the four bodies found at sea, one was that of a six-month-old baby, rescuers said. Nine other people were pulled from the water alive.
"The rescue operation for the two others is continuing," the coast guard official said. Initial reports suggested Iranians were also among the group, but the coast guard later confirmed this was not the case.
Under the draft deal struck on Monday, Turkey also received promises of faster visa liberalisation for Turks travelling to Europe, and a speeding up of Ankara's long-stalled EU membership talks.
The aim of the deal is to discourage refugees who trying to reach Europe illegally and break the grip of human smugglers who have sent them on perilous journeys across the Aegean.
However, refugees have continued to try to cross from Turkey's coast in recent days.
"Forming the mindset that 'You can't go there anymore, there's no hope there' is the basis of preventing (illegal migration)," Bozkir said on Thursday.
He also said Turkey would meet by May 1 the EU's conditions for visa-free travel to Europe.
More than 130,000 people this year have reached Europe from Turkey, which borders Iran and conflict zones in Syria and Iraq, while 350 people have died on the perilous journey, according to the International Organisation for Migration.
More than 35,000 people are trapped at the Greek-Macedonian border at Idomeni as Europe tries to close the Balkan route to foreigners fleeing violence and economic upheaval.