Greece’s coastguard said on Saturday that five refugees - four women and a child - drowned when their boat capsized off the Greek island Samos close to Turkey’s coast.
Officials said five other refugees were rescued and a helicopter was searching for more survivors.
The incident occurred 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.
A long awaited European Union deal with Turkey, aimed at decreasing the overwhelming passage of undocumented refugees into Europe, began in action on Monday, with Ankara launching the first return of refugees from the island of Lesbos to Turkey.
Under the EU-Turkey deal, Ankara will simultaneously welcome refugees from Greece into Turkey as the EU takes Syrian refugees from Turkey into EU countries at the same rate.
Some 5,600 refugees have been registered on Greek islands since March 20, the date on which the agreement with Turkey went into effect.
Turkish Deputy Prime Minister Mehmet Simsek said in January that the number of Syrian refugees in Turkey had reached the 2.4 million mark, more than in any other country according to UN records.