In one of Europe’s biggest maritime tragedies, at least 79 migrants drowned early Wednesday, and hundreds more were believed dead or missing when their overcrowded boat overturned and sank in open waters near Greece.

As the hunt for survivors proceeded, a European rescue support organization estimated that 750 people were on board the 20-30 metre (65-100 foot) vessel.

Greek officials said it was too early to guess on the figure.

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.