A fishing boat carrying migrants trying to reach Europe capsized and sank off Greece on Wednesday, authorities said, leaving at least 79 dead and many more missing in one of the worst disasters of its kind this year.

Coast guard, navy and merchant vessels and aircraft fanned out for a vast search-and-rescue operation set to continue overnight. It was unclear how many passengers might still be in the water or trapped in the vessel, but some initial reports suggested hundreds of people may have been on board.

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 the deepest area of the Mediterranean Sea — and such depths could hamper any effort to locate a sunken metal vessel.

Twenty-five survivors were hospitalized with symptoms of hypothermia.

At the southern port of Kalamata, around 70 exhausted survivors bedded down in sleeping bags and blankets provided by rescuers in a large warehouse, while outside paramedics set up tents for anyone who needed first aid.

Katerina Tsata, head of a Red Cross volunteer group in Kalamata, said the migrants were also given psychological support.

“They suffered a very heavy blow, both physical and mental,” she said.

The Greek coast guard said 79 bodies have been recovered so far. It said the survivors included 30 people from Egypt, 10 from Pakistan, 35 from Syria and 2 Palestinians.

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 migration route from North Africa to Italy through the central Mediterranean is the deadliest in the world, according to the United Nations migration agency, known as IOM, which has recorded more than 17,000 deaths and disappearances there since 2014.

Smugglers use unseaworthy boats with as many migrants as possible crammed in — sometimes inside locked holds — for journeys that can take days. They head for Italy, which is directly across the Mediterranean from Libya and Tunisia, and much closer than Greece to the Western European countries that most migrants hope to eventually reach.

In February, at least 94 people died when a wooden boat from Turkey sank off Cutro, in southern Italy, in the worst Mediterranean sinking so far this year.

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

The 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 it carried 750 people — 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.

“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.

Over the last two years eastern Libya has seen a resurgence of smuggling operations involving large wooden boats or old fishing vessels that can carry hundreds of migrants, many of them from Egypt and Bangladesh. While these boats are sturdier than inflatable rubber boats or other small craft used in western Libya and Tunisia, overcrowding makes the journey extremely dangerous.

The Mediterranean’s deadliest shipwreck in living memory occurred on April 18, 2015, when an overcrowded fishing boat collided off Libya with a freighter trying to come to its rescue. Only 28 people survived. Forensic experts concluded in 2018 that there were originally 1,100 people on board.

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

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

Separately Wednesday, a yacht with 81 migrants on board was towed to a port on the south coast of Greece's island of Crete after authorities received a distress call.