At least 79 migrants died early on Wednesday and many dozens were feared missing after a fishing boat carrying scores of people capsized off the Greek coast.
About 100 people were rescued and taken to the town of Kalamata following the incident in international waters in the Ionian Sea, the Greek coastguard said.
Coastguard authorities announced that the boat had previously declined several assistance offers. Both the coastguard and nearby merchant ships began their rescue efforts on Tuesday, according to an official statement.
The ship's captain, it was stated, expressed an intention to proceed to Italy.
Contrary to the coastguard's account, Alarm Phone, an activist network operating a hotline for distressed migrant boats, stated that it had been communicating with people presumed to be passengers on the capsized vessel.
These passengers conveyed an urgent need for aid, in stark contrast to the ship captain's earlier refusal.
Alarm Phone also claimed that the captain had abandoned the ship using a smaller boat prior to the vessel capsizing. The assertions made by the organisation are currently under investigation.
Along with navy vessels, the rescue operation included an army plane and helicopter, a drone from the EU border protection agency Frontex, as well as six other boats that were in the area. Dozens of survivors were able to board a superyacht, the Mayan Queen IV.
The spot is close to one of the deepest areas of the Mediterranean Sea and the rescue was complicated by strong winds, the coastguard said. “We fear the number of dead will rise,” said a shipping ministry official.
Alam Phone, a European rescue support charity, said it believed there were 750 people on board the 20- to 30-metre-long vessel.
Four of the survivors were taken to hospital with symptoms of hypothermia.
Katerina Tsata, head of a Red Cross volunteer group in Kalamata, said: “They suffered a very heavy blow, both physical and mental.”
Coastguard spokesman Nikos Alexiou said it appeared that the ship had capsized after people abruptly moved to one side.
“The outer deck was full of people, and we presume that the interior [of the vessel] would also have been full,” he said. “It looks as if there was a shift among the people who were crammed on board and it capsized.”
Ioannis Zafiropoulos, deputy mayor of Kalamata, said: “It sank very quickly and was gone by the time the rescue helicopter got there. The area where this happened has very deep water.”
State broadcaster ERT said the Italy-bound boat had sailed from the Libyan town of Tobruk, which lies south of the Greek island of Crete. Most of those on board were young men in their 20s.
Their nationalities, as well as where the boat had sailed from, were not immediately confirmed by Greek authorities.
The coastguard said the boat was first spotted in international waters late on Tuesday by an aircraft belonging to EU border agency Frontex and two nearby vessels, about 80km south-west of the town of Pylos in southern Greece.
It said those on board had refused assistance offered by Greek authorities. A few hours later the boat capsized and sank, triggering the search and rescue operation.
Alarm Phone said it had been contacted by people on a boat in distress on Tuesday afternoon. That boat was in the same general area as the one that sank, but it was not clear if it was the same vessel.
The organisation notified Greek authorities and Frontex. In one communication with Alarm Phone, migrants reported the vessel was overcrowded and that the captain had abandoned the ship on a small boat, according to the group. They asked for food and water, which was provided by a merchant ship.
Greece, Italy and Spain are the main destinations for the tens of thousands of people seeking to reach the Europe from Africa and the Middle East.
Smugglers are increasingly taking larger boats into international waters off the Greek mainland to try to avoid coastguard patrols.
EU Commission President Ursula von der Leyen tweeted: “Deeply saddened by the news of the shipwreck off the Greek coast and the many reported deaths. Very concerned by the number of missing people.
“We must continue to work together, with member states and third countries, to prevent such tragedies.”
The United Nations has registered more than 20,000 deaths and disappearances in the central Mediterranean since 2014, making it the most dangerous migrant crossing in the world.
The worst migrant tragedy in Greece was in June 2016 when at least 320 people were listed as dead or missing, according to records going back to 1993.
The International Organisation for Migration as of Wednesday had listed 48 migrants missing in the Eastern Mediterranean so far this year, compared to 378 a year earlier.
Separately on Wednesday, Greece's port police said a sailing boat in distress carrying about 80 migrants off Crete was rescued by a coastguard patrol and towed to port.
Greece is facing an increase in crossing attempts from Turkey on southern routes near the Cyclades islands and towards the Peloponnese peninsula, hoping to avoid patrols in the northern Aegean Sea.
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.
