A sailboat carrying migrants sank in in the Aegean sea between Turkey and Greece on Tuesday and at least 17 people were missing, Greek authorities said.
Greece's maritime ministry said Greek and Turkish rescue efforts had saved 16 of the between 33 and 37 migrants believed to be on board when the boat went down between the Greek islands of Farmakonisi and Agathonisi.
Their nationalities were not known.
"The operation is ongoing," said a spokeswoman for Greece's maritime ministry. "We are still looking for 17 to 21 people."
More than 135,000 refugees and migrants have arrived in Europe by sea in the first half of this year, many of them fleeing war, repression and poverty in Africa and the Middle East. Most are Syrians.
Greece is the largest arrival point, putting a huge strain on the country as it wrestles with one of the worst economic crises of modern times.