Up to 100 children are believed to have been in the hold of an overcrowded ship that sank off the coast of Greece, with hundreds of people feared dead.

As many as 750 migrants may have been crammed onto the fishing vessel that capsized in deep waters about 80 kilometres from Greece, .

Greek authorities confirmed 78 people were killed and 104 survivors had been brought ashore as rescuers scour the seas.

But hopes of finding more survivors are dwindling as fears grew that hundreds more, including children, may have drowned inside the cramped vessel’s hold.

The BBC said survivors had spoken of 50 or 100 children on board in one of Europe’s worst migrant disasters.

The senior doctor at Kalamata General Hospital, Dr Manolis Makaris, told the BBC that women and children were below the deck.

He said two patients had given him estimated figures.

“One told me about 100 children, the other about 50, so I don’t know the truth – but it is many,” he said.

The ship was heading to Italy after leaving the port of Tobruk in Libya when it began to flounder late on Tuesday night (local time).

Pople on the crowded outer deck repeatedly turned down offers of help from a Greek coast guard boat that was shadowing it, saying they wanted to reach Italy, according to Greek authorities.

“When you are faced with such a situation… you need to be very careful in your actions,” coast guard spokesman Nikos Alexiou told state broadcaster ERT.

“You cannot carry out a violent diversion on such a vessel with so many people on board … without any sort of co-operation.”

As Greece declared three days of mourning, bodies of the victims were transferred to a cemetery near Athens for DNA tests.

The search operation had not recovered any bodies in the past 24 hours but the coast guard said it would continue for as long as needed.

Women and children were reportedly under the deck of the crowded fishing boat that sank off Greece. Photo: Hellenic Coast Guard

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.

Authorities were investigating an account from a European rescue-support charity that there could have been 750 people on the 20- to 30-metre-long boat.

The United Nations’  International Organisation for Migration said initial reports suggested up to 400 people were aboard.

Its refugee agency, the UNHCR, said hundreds were feared missing.

“The shipwreck off Pylos marks one of the largest sea tragedies in the Mediterranean in recent memory,” said Maria Clara, the UNHCR representative in Greece.

Pope Francis, who visited Greece two years ago to draw attention to the plight of migrants and refugees was “deeply dismayed to learn of the shipwreck … with its devastating loss of life,” the Vatican said.

Of the 104 survivors so far transferred by the coast guard to the Greek port city of Kalamata, most were men, authorities said.

They revised their overnight death toll to 78 from 79.

But government sources said chances of retrieving the sunken vessel were remote because of the depth of the water.

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.

It said it had alerted Greek authorities and spoke to people on the vessel who appealed for help, and that the captain had fled on a small boat.

Government officials said that before capsizing and sinking about 2am on Wednesday, the vessel’s engine stopped and it began veering from side to side.

Independent refugee activist Nawal Soufi said in a Facebook post that she was contacted by migrants aboard the vessel in the early hours of Tuesday, and that she had been in contact with them until 11pm.

“The whole time they asked me what they should do and I kept telling them that Greek help would come. In this last call, the man I was talking to expressly told me: ‘I feel that this will be our last night alive’,” she wrote.

On Thursday, a senior prosecutor took over the supervision of the investigation launched by coast guard authorities over the incident, state broadcaster ERT reported.

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