A fishing boat crammed to the gunwales with migrants trying to reach Europe capsized and sank Wednesday off the coast of Greece, 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 were missing, but some initial reports suggested hundreds of people may have been aboard when the boat went down far from shore.

An aerial photograph of the battered blue vessel released by the Greek coast guard showed scores of people covering practically every inch of deck.

Greece’s caretaker prime minister, Ioannis Sarmas, declared three days of national mourning, “with our thoughts on all the victims of the ruthless smugglers who exploit human unhappiness.

Coast guard spokesman Nikos Alexiou told state ERT TV that it was impossible to accurately estimate the number of passengers. He said it appeared that the 25- to 30-meter (80- to 100-foot) vessel 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.”

A coast guard statement said efforts by its own ships and merchant vessels to assist the boat were repeatedly rebuffed, with people on board insisting they wanted to continue to Italy. Coast guard officials said the trawler’s engines broke down around 1:40 a.m. Wednesday, and just under an hour later, the ship started to list abruptly from side to side before capsizing.

The ship sank 10 to 15 minutes later, the statement said.

Ioannis Zafiropoulos, deputy mayor of the southern port city of Kalamata, where survivors were taken, said that his information indicated there were “more than 500 people” on board.

Authorities said 104 people were rescued after the sinking in international waters about 75 kilometers (45 miles) southwest of Greece’s southern Peloponnese peninsula. The spot is close to the deepest part of the Mediterranean Sea, and depths of up to 17,000 feet (5,200 meters) could hamper any effort to locate a sunken vessel.

Twenty-five survivors ranging in age from 16 to 49 were hospitalized with hypothermia or fever.

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

Rescue volunteer Constantinos Vlachonikolos said nearly all the survivors were men.

“They were very worn out. How could they not be?” he said. Rescuers said many of the people pulled from the water couldn’t swim and were clutching debris. The coast guard said none had life jackets.

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

The Italy-bound boat was believed to have left the Tobruk area in eastern Libya — a 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 instability, and made Libya one of the main departure points for people attempting to reach Europe on smuggler’s boats.

The route from North Africa to Italy through the central Mediterranean is the deadliest in the world, according to the U.N. migration agency, known as IOM, which has recorded more than 21,000 deaths and disappearances there since 2014.

Smugglers use unseaworthy boats and cram as many migrants as possible inside — 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 wasn’t 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, and 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,” the coast guard said. A second merchant ship later offered more supplies and assistance, which were turned down, the agency 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 and later headed a major rescue operation by all the ships in the area.

Alarm Phone, a network of activists that provides a hotline for migrants in trouble, 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 were provided by a merchant ship.

“We fear that hundreds of people have drowned,” Alarm Phone said in a statement.

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 that there were originally 1,100 people on board.