At least 18 migrants died on Sunday after their boat capsized in the Aegean Sea while trying to cross from Turkey to Greece, a Turkish official said.
The Turkish coastguard rescued 15 migrants from the boat off the coastal town of Didim in southwest Turkey, the official told AFP, speaking on condition of anonymity.
The nationality of the migrants was not immediately known.
The coastguard, backed up by three boats and one helicopter, was hunting for the missing, the state-run Anatolia news agency reported.
The tragedy comes a day before a crucial summit between Turkey and European Union leaders in Brussels.
Turkey is the main launch pad for most of the more than one million refugees and migrants who have crossed into Europe since early 2015 and under is pressure to curb the huge flow.
The 28-nation bloc is hoping for new commitments from Turkish Prime Minister Ahmet Davutoglu at their talks in Brussels in order to stop the stream of refugees.
In an official visit to Ankara this week, EU President Donald Tusk said the number of migrants crossing the Aegean Sea from Turkey to EU territory remained "far too high".