Athens: At least 79 migrants drowned early on Wednesday and hundreds more were feared dead or missing after their overloaded boat capsized and sank in open seas off Greece, in one of Europe's deadliest shipping disasters.

As a painstaking search for survivors continued, a European rescue support charity said it believed around 750 people were on board the 20-30 metre-long (65-100 foot) vessel.

Greek authorities said it was too soon to speculate on the number.

Alarm Phone, which operates a trans-European network supporting rescue operations, said it received alerts from people on board a ship in distress off Greece late on Tuesday, subsequently losing contact.

"According to the people, there were 750 people on board... We now hear reports of a shipwreck and fear they are true," it said on Twitter.

The U.N.'s International Organization for Migration said in a tweet that initial reports suggested up to 400 people were on the boat.

Greek authorities said it was unclear how many the vessel was carrying when it went under, and that 104 people had been rescued by midday.

"It is not safe to give a number. We do not know how many people were in the hold," coast guard spokesperson Nikos Alexiou told Greece's MEGA TV.

"...There were too many people on the outer deck. It was full."

The boat's occupants had refused an offer of help late on Tuesday, the coast guard added.

Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa. Most cross over to Greek islands from nearby Turkey.

But since the previous conservative government of Kyriakos Mitsotakis introduced tougher surveillance at the country's migrant camps, increasing numbers have chosen to make a longer, more dangerous journey from Turkey to Italy via Greece.

State broadcaster ERT said the boat that sank was en route for Italy, having set sail from the Libyan town of Tobruk, which lies south of the Greek island of Crete. Greek authorities did not confirm the vessel's departure port.

The shipwreck is the deadliest off Greece this year, and among the worst in Europe. In February, 96 people died when their wooden boat smashed into rocks on Italy's Calabrian coast during a storm.

The Greek migration ministry blamed international smuggling networks for putting migrants' lives at risk, while Filippo Grandi, the U.N. High Commissioner for Refugees, called on governments to work together on creating safe pathways for people fleeing poverty and war.

International waters
The Greek coast guard said the boat was first spotted late on Tuesday by EU border agency Frontex in international waters around 50 miles (80 km) southwest of the southern Greek coastal town of Pylos.

Italian authorities then alerted Greece to the vessel's presence, and the Greek coast guard approached it and offered help.

The large number of migrants on its outer deck "refused assistance and stated their desire to continue their voyage," the coast guard said.

A few hours later, the vessel began veering from side to side before capsizing around 2 a.m. on Wednesday and then sinking, a government official said.

Broadcaster ERT said most of those on board were young men in their 20s. A shipping ministry official who spoke on condition of anonymity said most were from Egypt, Syria and Pakistan.

Survivors were taken to the town of Kalamata and are expected to be moved from there to a camp outside Athens, the migration ministry said.

Greece was at the frontline of Europe's migration crisis of 2015-16, when nearly 1 million people arrived on its islands from Turkey before heading north to wealthier European states.

Numbers have fallen dramatically since a 2016 deal struck between Brussels and Ankara to stem the flows. Mitsotakis' government said its tougher stance, which also included more border patrols, had helped keep arrivals low.

Greece is currently led by a caretaker administration ahead of a national election on June 25 that the conservatives are expected to win.

About 72,000 refugees and migrants have arrived so far this year in Europe's frontline Mediterranean countries, according to United Nations data, with the majority landing in Italy and around 6,500 in Greece.

Nearly 1,000 people are estimated to have died or gone missing in the Mediterranean this year, according to the U.N.