At least 78 migrants drowned early on Wednesday with more feared missing or dead after a boat carrying them capsized and sank off the west coast of the Peloponnese region in Greece, the country's coast guard said.

The Hellenic Coast Guard published a statement saying that the boat, which was en route to Italy, was spotted approximately 50 miles (80 km) southwest of the town of Pylos in the southern Peloponnese.

The authorities reported that 104 people had been rescued and taken to the port town on Kalamata, but it still remains unclear how many were on board in total when the vessel capsized and sank.

The shipwreck is the deadliest to take place in Greek waters this year.

News of the incident prompted Kyriakos Mitsotakis, the leader of the Greek party New Democracy, to postpone Wednesday's campaign rally in the western port city of Patra in the Peloponnese.

Greece's caretaker prime minister, Ioannis Sarmas, declared three days of national mourning, "with our thoughts on all the victims of the ruthless smugglers who exploit human unhappiness."

Hundreds of migrants on the ship

Coast guard spokesman Nikos Alexiou told Greek state broadcaster ERT TV that it was impossible to estimate accurately the number of passengers. He said it appeared that the vessel capsized after people abruptly moved to one side. "The outer deck was full of people, and we presume that the interior [of the vessel] would also have been full," he said.

"It looks as if there was a shift among the people who were crammed on board, and it capsized."

Coast guard officials said the trawler's engines broke down around 1.40 am Wednesday, and just under an hour later, the ship started to list abruptly from side to side before capsizing. Minutes later the ship sank.

Ioannis Zafiropoulos, deputy mayor of the southern port city of Kalamata, where survivors were taken, said that his information indicated there were "more than 500 people" on board. Some witnesses maintained that many children were on board and trapped inside the ship's hold.

State broadcaster ERT said that the ship had sailed from the Libyan town of Tobruk, which lies south of the Greek island of Crete, with the majority on board "young men in their 20s."

However, the nationalities of those on board, as well as confirmation of where exactly the boat had sailed from, has not yet been confirmed by the Greek authorities.

NGO: numbers of dead will 'drastically increase' in coming days

Norwegian NGO Aegean Boat Report said that the numbers of the dead would "drastically increase" in the coming days, publishing the following statement about the tragic incident: "We expect numbers [of those who have drowned] to drastically increase when Greek authorities have updated their official information.

The Greek coast guard released a statement saying that Greek and Italian authorities together with Frontex were informed of the vessel on Tuesday and that several attempts were made to assist the fishing boat but the people onboard refused assistance, and wanted to continue towards Italy.

NGO Aegean Boat Report, often outspoken in its criticism of Greece's policies on migrants, questioned why the vessel was not stopped earlier in its journey to prevent such a tragic outcome of events.

"Why this overloaded fishing vessel wasn't stopped at an earlier stage, as they usually do, to avoid such tragedies, since they knew of its existence, is unknown," they said.

A large number of vessels in the area, together with coast guard and military, were involved to locate and rescue people, and searched for survivors and the drowned.

Another migrant boat tragedy off Greece

Greece has been in the spotlight in recent months following a series of other tragic shipwrecks that claimed the lives of migrants, including children.

On May 26 a shipwreck off the coast of the popular holiday island of Mykonos saw 9 migrants lose their lives. This followed an incident a month earlier, which cost the life of one migrant – 47 others were rescued.

Despite a reduction in the total number of asylum seekers in Greece, there have been significant increases in arrivals in 2023 so far compared with last year.

Figures released last month show a major increase in the number of irregular migrants arriving to the country year on year -- a rise of 96% in 2022 relative to 2021.

Attempts by undocumented migrants to cross into Greece on boats from Turkey have been rising since autumn 2022, and the Greek government continues to strengthen its security measures such as the increased number of patrols on both its sea and land borders.

Earlier this month Greece announced the approval of €30 million more of funding to strengthen its coast guard. This latest investment, which involves co-financing by the European Union from the Migration and Home Affairs Funds 2021-2027, will account for the purchase of two coastal patrol vessels with a total length of more than 30 meters.