Rescue ships set out on Thursday in search of hundreds of refugees after their overcrowded fishing boats capsized while trying to reach Europe and migrants were feared dead.
At least 79 people have been confirmed dead in this disaster.
Rescuers rescued 104 passengers from a boat that capsized in deep water off the coast of Greece while trying to cross from Libya to Italy early Wednesday.
Authorities fear that hundreds more – including women and children – may be trapped below deck.
If confirmed, it would make it the worst tragedy on record in the central Mediterranean.
“Probability of detection [more survivors] At least,” retired Greek Coast Guard Admiral Nikos Spanos told state-run ERT television.
The International Organization for Migration estimated that there were 700 to 750 people on board, including at least 40 children, based on interviews with survivors. Save the Children puts the number higher, around 100 children.
Reports say the boat sank about 80 kilometers (50 mi) off the southern coastal town of Pylos.
Refugee activists, refugee rescue NGOs, some European politicians and Pope Francis were among those who shared their sadness and anger at the disaster.
The Vatican said the pontiff was “deeply disappointed” and offered “fervent prayers for the many migrants who died, their loved ones and all those injured in this tragedy”.
‘Should be a wake-up call for EU governments’
Aerial images released by the Greek coast guard showed dozens of people on the upper and lower decks of the boat looking up, some with arms outstretched, hours before it sank.
Alarm Phone, which operates a trans-European network helping rescue operations, said it received an alert late Tuesday from people aboard a ship in distress near Greece.
It said it alerted Greek authorities and spoke to people on board who pleaded for help and the captain escaped in a small boat.
Government officials said that the ship’s engine stopped and it began to roll back and forth before it capsized around 2 a.m. Wednesday.
Most of the survivors evacuated to the port city of Kalamata so far are men, authorities said.
Al Jazeera’s John Saropoulos, reporting from Kalamata, said doctors treated several survivors, dozens of whom had “near-drowning” symptoms.
“They were suffering from a type of pneumonia that occurs when the lungs are partially flooded with water,” he said. “Some are sent back to a warehouse while others survive.”
As Greece declared three days of mourning, the bodies of the victims were transferred to a cemetery near Athens for DNA testing. The Coast Guard said the search operation will continue for as long as necessary.
“[European Union] Member states have gone to extraordinary lengths to block all routes for children and their families to safety in Europe. Often their only option is to make the dangerous journey by boat,” said Daniel Gorevan, Senior Advocacy Advisor at Save the Children.
“The continued death of people in the Mediterranean should be a wake-up call for EU governments,” he warned.
Official sources said the chances of the sunken ship sailing from the Libyan port of Tobruk were remote. The area of international waters where the incident occurred is one of the deepest areas of the Mediterranean Sea.
Independent refugee activist Nawal Soufi said in a Facebook post that he had been contacted by people on the ship early on Tuesday and that he had been in contact with them until 11pm (20:00 GMT).
“The whole time they asked me what they should do and I kept telling them that Greek help would come. On this last call, the person I spoke with told me bluntly: ‘I feel like this will be our last night alive,'” he wrote.
Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.
Greece, under the conservative government in power until last month, has taken a tough stance on immigration, building walled camps and increasing border controls.
The country is currently governed by a caretaker administration pending an election on June 25.
Libya, which has had little stability or security since a NATO-backed uprising in 2011, is a major launching point for those seeking to reach Europe by sea, with its people-smuggling network run mainly by military groups that control coastal areas.
The UN has registered more than 20,000 deaths and disappearances in the central Mediterranean since 2014, making it the world’s most dangerous migrant crossing.