A fishing boat carrying migrants capsized and sank off the coast of Greece on Wednesday, authorities said, leaving at least 78 people dead and many dozens feared missing in one of the worst disasters of its kind this year.

Coast guard, navy, and merchant vessels fanned out for a vast search-and-rescue operation that also included a plane and a helicopter.

Authorities said 104 people have been rescued so far after the boat sank overnight in international waters some 45 miles southwest of Greece’s southern Peloponnese peninsula. The spot is close to one of the deepest areas of the Mediterranean Sea.

It was unclear how many passengers might still be in the water or trapped in the sunken vessel, but some initial reports suggested hundreds of people may have been on board.

Four survivors were hospitalized with symptoms of hypothermia. At the southern port of Kalamata, dozens of others were taken to sheltered areas set up by the ambulance services and the United Nations Refugee Agency to receive dry clothes and medical attention.

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The Greek coast guard said 78 bodies have been recovered so far.

The Italy-bound boat is believed to have sailed from the Tobruk area in eastern Libya. That country plunged into chaos following a NATO-backed uprising that toppled and killed longtime autocrat Moammar Gadhafi in 2011. Human traffickers have benefited from the ensuing instability, and made Libya one of the main departure points for people attempting to reach Europe on smuggler’s boats.

The Italian coast guard first alerted Greek authorities and the European Union border protection agency, Frontex, about the approaching vessel on Tuesday.

The United Nations migration agency, IOM, said initial reports suggested up to 400 people were on board. A network of activists said it received a distress call from a boat in the same area whose passengers said 750 people were on board — but it was not clear if that was the vessel that sank.

After that first alert, Frontex aircraft and two merchant ships spotted the boat heading north at high speed, according to the Greek coast guard. More aircraft and ships were sent to the area.

But repeated calls to the vessel offering help were declined, the coast guard said in a statement.

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"In the afternoon a merchant vessel approached the ship and provided it with food and supplies, while the (passengers) refused any further assistance," it said. A second merchant ship that approached it later offered further supplies and assistance, which were turned down, it added.

In the evening, a coast guard patrol boat reached the vessel "and confirmed the presence of a large number of migrants on the deck," the statement said. "But they refused any assistance and said they wanted to continue to Italy."

The coast guard boat accompanied the migrant vessel, which, the statement said, capsized and sank early Wednesday, prompting a massive rescue operation by all the ships in the area.

Alarm Phone, a network of activists that provides a hotline for migrants in trouble, meanwhile, said it was 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 organization 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.

When they have larger boats, Mediterranean smugglers are increasingly trying to stay in international waters as they pass by Greece to try to avoid its coast guard patrols.

On Sunday, 90 migrants on a U.S.-flagged yacht were rescued in the area after they made a distress call.

Six Greek coast guard vessels, a navy frigate, a military transport plane, an air force helicopter, several private vessels and a drone from Frontex are currently taking part in the search for the boat that sank Wednesday.

Greek President Katerina Sakellaropoulou headed for the area where rescued migrants are being tended to and planned campaign events by political parties ahead of June 25 national elections were called off.

Separately Wednesday, a yacht with 81 migrants on board was towed to a por