At least 59 people died in a migrant boat capsize overnight Tuesday-Wednesday off the Peloponnese peninsula in southwestern Greece, according to the latest toll from the Coast Guard.

The boat, which was carrying "hundreds" of migrants, capsized 47 nautical miles from the Greek coast in international waters, according to a source in the Greek Ministry of Immigrants.

During the massive rescue operation that began early Wednesday morning, 104 people were rescued, including four who were taken to hospital in Kalamata, a town in the southern Peloponnese.

The Greek coast guard said that when the boat sank, none of the people on board, whose nationality has not been revealed, had a lifejacket.

A statement from Greek port authorities said the boat was first spotted on Tuesday afternoon from a Frontex plane, the European border control agency, but that the migrants on board "refused any help".

In addition to the port police patrol boats, a Greek Navy frigate, an Air Force plane and helicopter, and six boats sailing in the area participated in the rescue operation.

According to preliminary information, the boat carrying migrants from Libya to Italy has sailed.

On the European Union's external border in the Mediterranean, Greece is a regular corridor for many seeking to migrate to the EU from neighboring Turkey.

Many drownings occur in the Aegean Sea, while Greece is regularly accused by NGOs and the media of sending back migrants seeking asylum to the European Union.

Besides this route, migrants also try to pass directly to Italy by crossing the Mediterranean Sea south of the Peloponnese peninsula or Crete.