Four women and a child have drowned after their boat capsized off the Greek island of Samos close to Turkey's coast.
Five other migrants were rescued and Greek coast guard vessels, which were helped by a helicopter, were searching for more survivors.
The incident happened a day after two ferries took more than 120 migrants back to Turkey from the Greek island of Lesbos, in the second round of returns under an EU deal with Ankara to halt mass migration across the Aegean Sea to Europe.
The accord, which came into force on Monday, aims to stem an uncontrolled influx into the European Union of people fleeing war and poverty in the Middle East and beyond, after more than one million arrived last year.
The United Nations refugee agency (UNHCR) and human rights groups say the accord may violate international law.
Although flows have slowed, migrants continue to reach the Greek islands. Greek authorities said 149 had arrived on Lesbos, Samos and Chios in the 24 hours to Friday morning.