One hundred people were rescued, authorities said, but it remained unclear how many people were on board when the vessel sank. Survivors were taken to the town of Kalamata, authorities said.

The coast guard said the boat, which was sailing to Italy, was first spotted in international waters on Tuesday by a Frontex helicopter and two nearby vessels, some 75km southwest of the town of Pylos in southern Greece.

It said the migrants on board had refused assistance offered by the Greek authorities, and a search and rescue operation was launched in the early hours today after the boat capsized and sank.

Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa. Most cross to Greek islands from nearby Turkey, but a growing number of boats also undertake a longer, and more dangerous journey from Turkey to Italy through Greece.

The nationalities of the migrants, as well as where the boat had sailed from, were not immediately known.