THESSALONIKI, Greece (AP) — Four people believed to be migrants have been found dead on a crippled boat in the northern Aegean Sea and another 15 survivors have been rescued, Greek authorities said Wednesday.

The boat was located following a 17-hour search and rescue operation by sea and air, after somebody believed to be a passenger on the inflatable dinghy made a distress call to Greek officials late Tuesday.

All the passengers on the boat were men, but their nationalities or point of departure were not immediately known, the Merchant Marine Ministry said.

The ministry could not provide any information on how the victims died or how many people had been on the boat when it started its journey — most likely from the Turkish coast.

A coastguard vessel found the boat east of the island of Thassos, and the ministry initially said three of its passengers were unconscious. When the migrants were taken to the northern port town of Kavala later Wednesday, doctors pronounced them dead, together with a fourth man.

More than 170,000 people have reached Greece by sea from neighboring Turkey this year, most before a series of European border closures in March that stopped their further migration towards Europe's prosperous heartland.

Some 61,000 remain trapped in financially-crippled Greece, and most want to be relocated to another European country.