By midday, 104 people had been rescued, but it remained unclear how many were on board when the vessel went under, authorities said.

“We fear the number of dead will rise,” said a shipping ministry official who spoke on condition of anonymity.

The boat was first spotted late on Tuesday by EU border agency Frontex in international waters around 50 miles (80 km) southwest of the southern Greek coastal town of Pylos.

A coast guard vessel then approached the boat, which was en route to Italy, and offered help.

The large number of migrants on its outer deck “refused assistance and stated their desire to continue their voyage,” the coast guard said.

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.

Greek authorities did not immediately confirm its port of departure but the shipping ministry official said most of those on board were from Egypt, Syria and Pakistan.

Survivors were taken to the town of Kalamata.

Greece, currently led by a caretaker government ahead of a national election on June 25, 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.

(Reporting by Karolina Tagaris; Editing by Sharon Singleton, John Stonestreet)