At least 59 migrants have drowned and more are feared missing after their boat capsized and sank off Greece, the country’s coast guard says.

More than 100 people were rescued but it remained unclear how many were on board when the vessel went under, authorities said on Wednesday.

The shipwreck was the deadliest off Greece this year.

The coast guard said the boat, 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, about 80 kilometres south-west 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 onboard young men in their 20s.

Their nationalities, and where the boat had sailed from, were not immediately confirmed by Greek authorities.

Survivors were taken to the town of Kalamata, 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 year in Europe’s frontline countries Italy, Spain, Greece, Malta and Cyprus, according to United Nations data, with most of them landing in Italy.