One hundred people were rescued, but it remained unclear how many were on board when the vessel went under, authorities said. The shipwreck was the deadliest off Greece this year.

The coast guard said the boat, which was en route to Italy, was spotted in international waters late on Tuesday by an aircraft belonging to EU border agency Frontex and two nearby vessels, around 50 miles (80 km) southwest of the town of Pylos in southern Greece.

It said those on board had refused assistance offered by Greek authorities late on Tuesday. A few hours later the boat capsized and sank, triggering a search and rescue operation.

State broadcaster ERT said it had sailed from the Libyan town of Tobruk, which lies south of the Greek island of Crete, with most on board being young men in their 20s. Their nationalities, as well as where the boat had sailed from, were not immediately confirmed by Greek authorities.

Survivors were taken to the town of Kalamata, the authorities said.

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 via Greece.

About 72,000 refugees and migrants have arrived so far this years in Europe's frontline countries Italy, Spain, Greece, Malta and Cyprus, according to United Nations data, with the majority landing in Italy.