The death toll from the sinking of a fishing boat crammed with migrants off the coast of southwestern Greece has risen to 79, local media reported.

At least 104 migrants were rescued following the incident on Wednesday, but the number of deaths is feared to rise as survivors said the vessel was carrying up to 700 migrants, mostly from Pakistan, Egypt and Syria, said Greek public broadcaster ERT.

Some of the survivors said that many women and children were among the passengers. The Greek Coast Guard noted that it is difficult to determine exactly how many migrants were on board.

Greek authorities are investigating three survivors on the suspicion of being human traffickers, who drew attention with their relaxed attitude after being rescued and brought to the city of Kalamata.

All of those rescued will be interviewed as part of a preliminary investigation to shed light on the causes of the tragedy, said ERT.

Along with navy vessels, the rescue operation in international waters included an army plane and helicopter as well as six other boats that were in the area.

"Since very early Wednesday, an extensive rescue operation is underway off Pylos, after a fishing boat capsized with a large number of migrants onboard," the coastguard said.

The coastguard said a surveillance plane with Europe's Frontex agency had spotted the boat Tuesday afternoon, but the passengers "refused any help".

It later said that none on board were wearing life jackets and did not immediately disclose their nationalities. Authorities said it appeared the migrants had departed from Libya and were heading for Italy.

Increased crossing attempts

Also on June 14, Greece's port police said a sailboat in distress carrying about 80 migrants off Crete was rescued by a coastguard patrol and towed to port.

Greece along with Italy and Spain have long been the main landing points for the tens of thousands of people seeking to reach Europe from Africa and the Middle East.

Rescue operations are common, but last month the Greek government came under international pressure over video footage reportedly showing the forceful expulsion of migrants who were set adrift at sea.

Greece and other EU member states on the south and southeastern rim of the bloc say they are being unfairly tasked with managing arrivals of migrants.