At least 78 migrants have drowned and more than 100 have been rescued after their fishing vessel capsized off the coast of southern Greece.

The shipwreck is the deadliest off Greece this year.

It is unclear how many people were on the boat when it sank about 50 miles (80 km) south-west of Pylos.

But Greek officials and the International Organization for Migration (IOM) say that hundreds of people could have been on board.

Greece's coastguard said the boat was spotted in international waters late on Tuesday by an aircraft belonging to EU border agency Frontex.

The coastguard said that the passengers "refused any help" and that none on board were wearing life jackets.

But just a few hours later, the boat capsized and sank, triggering a search and rescue operation, which was then complicated by strong winds.

The boat was reportedly en route to Italy from Libya, with most of those on board reportedly men in their 20s.

A migration ministry source told the AFP news agency that "hundreds" of people were on board. "We fear there will be a very large number of missing persons," the official said.

The IOM tweeted: "We fear more lives were lost. Initial reports suggest up to 400 people were on board."

The nationalities of the victims have not yet been announced. Survivors have been taken to the town of Kalamata for treatment.

Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.

Last month the Greek government came under international criticism over video footage reportedly showing the forceful expulsion of migrants who were set adrift at sea.

More than 70,000 refugees and migrants have arrived in Europe's frontline countries this year, with the majority landing in Italy, according to UN data.