At least 79 people have died after a migrant boat carrying hundreds of people sank off the Greek coast in the early hours of Wednesday.

The Coast Guard said hundreds are feared missing in the deadliest migrant shipwreck adding that most of the victims were from Afghanistan and Pakistan.

The boat drowned when the large fishing vessel they were travelling in capsized off the southern Peloponnese, the Guardian UK.

It was unclear how many passengers were missing from the vessel, which had set out from eastern Libya and was heading for Italy.

“There has been a dramatic rise in the death count, which is climbing by the hour,” said one official. 

“Speculation is rife that as many as 600 people were onboard but that has not been confirmed. The ship is under the water. It has sunk.”

About 104 passengers had been rescued as of Wednesday afternoon, he said.

Ioannis Zafiropoulos, the deputy mayor of the southern port city of Kalamata, where survivors were taken, said his information indicated there were more than 500 people onboard. 

A European rescue support charity said it believed about 750 people were onboard, and the UN’s migration agency cited an estimate of as many as 400.

Coastguard vessels, a navy frigate, military transport planes, an air force helicopter and an array of private craft were participating in the search for survivors. Rescue efforts were initially hampered by strong winds.

Greek media outlets quoted survivors as saying the ship went down almost instantaneously, close to the deepest area of the Mediterranean Sea. “The engine stopped and it sunk in minutes,” said one.

On Wednesday evening the first aerial pictures emerged of the overcrowded vessel before it capsized. The blue trawler appeared to have hundreds of people on board.

“The outer part of the boat was full of people and we assume that was the case below deck too,” Nikos Alexiou, a spokesperson for the Hellenic coastguard, told reporters. 

“The boat was overcrowded. A precise number cannot with certainty be given but what is sure is that it is very big.”

Greek authorities and officials from the EU border agency Frontex were alerted to the stricken ship late on Tuesday. 

Repeated calls to the vessel offering help were declined, the coastguard said in a statement. “In the afternoon, a merchant vessel approached the ship and provided it with food and supplies, while the [passengers] refused any further assistance,” the coastguard said.

A second merchant ship later offered further supplies and assistance, which were turned down, the agency added. A coastguard patrol boat reached the vessel in the evening “and confirmed the presence of a large number of migrants on the deck”, the statement said. “But they refused any assistance and said they wanted to continue to Italy.”

Smugglers are taking ever-greater risks to evade patrols. They are increasingly operating in international seaways with the aim of dropping off their human cargo in Italy rather than heavily guarded Greece.