With at least 78 refugees dead and hundreds more feared missing after the sinking of a rusty trawler off Greece's Peloponnese peninsula, questions are being asked if more could have been done to prevent the tragedy.

What the Greek authorities say

The Greek coastguard and government officials say their patrol boats and nearby cargo ships had been shadowing the fishing boat since Tuesday afternoon, after it was spotted by a surveillance plane from Europe's Frontex agency.

They said the trawler had briefly stopped to take on food and water from a Maltese-flagged ship, but that a person on board, speaking English through a satellite phone, had insisted that no further assistance was needed and that those on board wished to continue their journey to Italy.

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"From (12:30 GMT to 18:00 GMT) the merchant marine operations room was in repeated contact with the fishing boat. They steadily repeated that they wished to sail to Italy and did not want any contribution from Greece," the coastguard said.

At 22:40 GMT, the trawler notified Athens of engine failure and the trawler stopped moving. The nearby patrol boat "immediately tried to approach the trawler to determine the problem," the coastguard said.

Twenty-four minutes later, the Greek patrol boat skipper radioed in that the boat had capsized. It sank within 15 minutes at 2:19 am Greek time.

How did the vessel sink?

There are mounting questions as to whether the Greek coastguard should have intervened earlier to escort the aged trawler, clearly packed with people, to safety.

Government spokesman Ilias Siakantaris said there were unconfirmed reports that up to 750 people had been on the boat. But the coastguard spokesman suggested the boat might have capsized earlier if they had attempted to intervene.

"You cannot divert a boat with so many people on board by force unless there is cooperation," he said.

Shahbaz from Pakistan, whose brother was on board completely refuted the idea that the authorities tried to help telling CGTN that five coastguard vessels didn't do anything to help until the boat capsized.

Greece's leftist former prime minister Alexis Tsipras said the refugees had actually "called for help" after talking to survivors at the western port of Kalamata.

Government spokesman Siakantaris confirmed that a rope was thrown to "stabilize" the boat, but the refugees had refused help, saying, "No help, go Italy."

"There was never an attempt to tie the vessel, neither by us nor any other ship," the coastguard spokesman said.

What happened on board

AlarmPhone, which runs a hotline for migrants in distress at sea, said those on board had reported at 15:20 GMT on Tuesday that the captain had fled on a small boat.

Fourteen minutes later, the refugees said that "the boat is overcrowded and... moving from side to side."

This is around the time the Greek coastguard said an English speaker on board had insisted the vessel was "in no danger" and did not require assistance.

The NGO also noted that refugees are reluctant to be intercepted by Greek forces owing to widespread reports of mistreatment and pushbacks, which Athens constantly denies.

Delayed rescue operation

Vincent Cochetel, special envoy of the United Nations High Commissioner for Refugees for the central and western Mediterranean, said Greece's argument for not intervening "does not hold up."

"Under international law, Greek authorities should have organized this rescue operation sooner, as soon as Frontex spotted the boat in distress," he said. "The boat was full to bursting... and the photos taken by Frontex leave no doubt that it was adrift and that people were objectively in a distress situation," Cochetel said.

Early on Friday, survivor Mohammad, 18, from Syria, burst into sobs as he spotted his elder brother Fadi, who had traveled from the Netherlands searching for him.

They wept and hugged through metal barricades, erected by Greek police around a warehouse in Kalamata where survivors had been sleeping for the past two days.

"Thank God for your safety," Fadi said, repeatedly kissing his younger sibling on the head.

A massive search and rescue operation continued on Friday, but hopes were dwindling of finding any more survivors from the hundreds of people believed to have been on board the boat when it sank in some of the deepest waters of the Mediterranean.

UN calls for 'thorough' Investigation 

On Friday, the UN called for "thorough" investigations into the "horrific tragedy."

UN rights office spokesman Jeremy Laurence said there was a need to investigate "people smugglers and human traffickers and ensure they are brought to justice," and that more broadly "there are a lot of questions that need to be asked."

The UN agencies for refugees and migrants called in a joint statement for "urgent and decisive action to prevent further deaths at sea."

They insisted states have an obligation to come together to address the dangerous gaps in search and rescue operations in the Mediterranean.

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