At least seven illegal migrants died when their boat sank on Thursday off the Greek island of Lesbos, Greece's coastguard said.
The boat was detected by a patrolling coastguard vessel at about 02:00 (23:00 GMT) north of Lesbos.
So far 23 illegals have been rescued and a search and rescue operation was still ongoing, a coastguard official said. It was not immediately clear how many people may have been on the boat.
Greece, in the southeast corner of the European Union, has long been a favoured gateway to Europe for illegal migrants and so-called refugees from the Middle East, Africa and Asia. In 2015 nearly one million people landed on its islands.
Last year, about 54,000 migrants reached Greece, the second largest number in southern Europe behind Italy. The vast majority of them arrived by sea, according to data from the UN refugee agency UNHCR.
(Reporting by Angeliki Koutantou, Yannis Souliotis and Ivana Sekularac; Editing by Alison Williams)