KALAMATA, Greece Rescuers were scouring the seas off Greece on Thursday in a massive search operation as hopes dwindled for survivors of a shipwreck that killed at least 79 people, in one of Europe's deadliest such disasters in recent years.

Hundreds of people were thought to have packed a fishing boat that capsized in international waters about 50 miles (80 km) from the southern coastal town of Pylos early on Wednesday, while being shadowed by the Greek coast guard.

As dawn broke on Thursday, a coast guard vessel sailed into the nearby port city of Kalamata, transferring victims of the year's deadliest shipwreck off Greece. By the early hours, the survivors there numbered 104, while deaths stood at 79.

Authorities said it was unclear how many had been aboard the vessel, and they were investigating one account from a European rescue-support charity that there could have been 750 people on the 20- to 30 metre-long (65- to 100- foot-long) boat.

The U.N.'s International Organization for Migration said initial reports suggested up to 400 people were aboard.

Government officials said the vessel had set off from the Libyan port of Tobruk, but added that migrants on the boat had persistently refused offers of help from Greek authorities.

"It was a fishing boat packed with people who refused our assistance because they wanted to go to Italy," coast guard spokesperson Nikos Alexiou told broadcaster Skai TV.

"We stayed beside it in case it needed our assistance which they had refused."

Aerial pictures released by the Greek coast guard showed dozens of people on the boat's upper and lower decks looking up, some with arms outstretched, hours before it sank.

Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.

Under a conservative government, in power until last month, authorities have taken a harder stance on migration, building walled camps and boosting border controls.

Libya, which has had little stability or security since a NATO-backed uprising in 2011, is a major launching point for those seeking to reach Europe by sea.

People-smuggling networks are mainly run by military factions that control coastal areas.

The United Nations has registered more than 20,000 deaths and disappearances in the central Mediterranean since 2014, making it the most dangerous migrant crossing in the world.

(Reporting by Stamos Prousalis and Stelios Misinas; Writing by Michele Kambas; Editing by Clarence Fernandez)