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 to 30-metre-long vessel, while the United Nations’ (UN) migration agency cited an estimate of up to 400.

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

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.

Greek 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 island of Crete. Greek authorities did not confirm the vessel’s departure port.

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, but subsequently lost 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.

Greek authorities said it remained 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,” Greek coast guard spokesperson Nikos Alexiou told Greece’s MEGA TV. “… There were too many people on the outer deck. It was full.”

Late on Tuesday, a few hours before the boat capsized, the boat’s occupants refused an offer of help, insisting on continuing their journey, the coast guard said.

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 UN High Commissioner for Refugees, called on governments to work together on creating safe pathways for people fleeing poverty and war.