The incident occurred off Greece’s Agathonisi island, which is close to the Turkish coast. The identity and nationality of the victims was not immediately known.
“At least four more (migrants) were unaccounted (for),” a coast guard official told Reuters. Three others were rescued.
Saturday’s incident was thought to be the highest death toll of migrants trying to reach outlying Greek islands in months.
Greek authorities said they believed there were 22 people on the boat. Greek coast guard vessels assisted by two helicopters were searching for more survivors.
“We can’t tolerate to lose children in the Aegean Sea ... the solution is to protect people, to implement safe procedures and safe routes for migrants and refugees, to hit the human trafficking circuits,” Greek migration minister Dimitris Vitsas said in a press release.
Hundreds of thousands of refugees and migrants flowed into Greece and further west in 2015 from Turkey, making the short but precarious crossing. Hundreds died trying to make it across.
Following a 2016 agreement between the European Union and Turkey, the numbers have diminished.
Under that deal, anyone who arrives on Greek islands must be returned to Turkey unless they qualify for asylum. But procedures can take several months, leading to overcrowded conditions on the islands.