It was not clear where the boat was headed or where it left from. Hundreds of thousands of migrants crossed the sea channel from Turkey to Greek territory in 2015 before Ankara curbed the flow under a deal it struck with the European Union.
The coast guard said the boat sank off the coast of Turkey's Izmir province after water began leaking shortly after its departure. Search and rescue operations for the missing migrants were continuing, a coast guard statement said. It said there were initially around 35 migrants on the boat in total.
Turkey became one of the main launch points for more than a million migrants taking the sea route to EU territory in 2015, many fleeing conflict and poverty in the Middle East and Africa.
Mediterranean arrivals to the bloc, including refugees making the longer and more perilous crossing from North Africa to Italy, totalled 172,301 in 2017, down from 362,753 in 2016 and 1,015,078 in 2015, according to United Nations data.
(With inputs from agencies.)