ATHENS, Greece -- At least 78 migrants died after their fishing boat sank off the Peloponnese, Greece's coastguard said Wednesday, as fears mounted that the death toll could rise to hundreds.

Some 100 people were rescued after the boat capsized in international waters in the Ionian Sea in an operation complicated by strong winds, the coastguard said.

A migration ministry source told AFP that according to the coastguard there could have been "hundreds" of people on the fishing boat.

"We fear there will be a very large number of missing persons," the official said.

The International Organization for Migration (IOM) acknowledged fears of a large number of missing people, noting in a tweet: "We fear more lives were lost. Initial reports suggest up to 400 people were onboard."

Other Greek officials said survivors had claimed up to 750 people may have been crammed on the fishing boat.

Greece's head of state, President Katerina Sakellaropoulou, immediately travelled to the port of Kalamata to confer with senior officials on the rescue and accommodation response.

Greece is currently under an interim government until June 25 elections. The leaders of the two main parties, former conservative prime minister Kyriakos Mitsotakis and leftist rival Alexis Tsipras, suspended campaign speeches scheduled for Wednesday.

The worst migrant tragedy in Greece was in June 2016 when at least 320 people were listed as dead or missing, according to AFP records going back to 1993.

The IOM until Wednesday had listed 48 migrants missing in the eastern Mediterranean so far this year, compared to 378 a year earlier.

Along with navy vessels, Wednesday's rescue operation included an army plane and helicopter as well as six other boats that were in the area.

"Since very early Wednesday, an extensive rescue operation is underway off Pylos, after a fishing boat capsized with a large number of migrants onboard," the coastguard said.

- Passengers 'refused help' -

Greek media reported that 26 people have been taken to the port's hospital, among them four in a more serious condition who had to be airlifted by helicopter.

The coastguard said a surveillance plane with Europe's Frontex agency had spotted the boat on Tuesday afternoon, but the passengers had "refused any help".

It added that none on board were wearing life jackets, and did not immediately disclose their nationalities.

Authorities said it appeared the migrants had departed from Libya and were heading for Italy.

The migration ministry said it had sent extra staff to Kalamata, including translators, and that the survivors would be transferred to a migrant camp near Athens.

The sinking "brings to the fore once again, in the most tragic way, the need to dismantle the international smuggling rings that put the lives of migrants in danger," the ministry said in a statement.

Also Wednesday, a sailing boat in distress carrying about 80 migrants off Crete was rescued by a coastguard patrol and towed to port, Greece's port police said.

Along with Italy and Spain, Greece has been one of the main landing points for tens of thousands of people seeking to reach Europe from Africa and the Middle East.

Greece is also facing an increase of crossing attempts from Turkey on southern routes near the Cyclades islands and toward the Peloponnese peninsula, hoping to avoid patrols in the northern Aegean Sea.

Rescue operations are common, but last month the Greek government came under international pressure over video footage reportedly showing the forceful expulsion of migrants who were set adrift at sea.

Greece and other EU member states on the southern and southeastern rim of the bloc say they are being unfairly tasked with managing arrivals of undocumented migrants.

© Agence France-Presse