ATHENS, Greece >> The Greek coast guard today defended its response to a ship that went down off the country’s south coast and left more than 500 migrants presumed drowned, as criticism mounted over the years-long failure to hammer out a comprehensive policy on migration into Europe.

Patrol boats and a helicopter spent a third day scouring the area of the Mediterranean Sea where the packed fishing vessel capsized early Wednesday, in what the U.N. migration agency said could be the second deadliest shipwreck recorded. The deadliest occurred when a vessel capsized off the coast of Libya en route to Italy in April 2015, killing an estimated 1,100 migrants.

Greek Coast guard spokesman Nikos Alexiou said that both coast guard and private ships repeatedly offered by radio and loudspeaker to help the vessel Wednesday while it was in international waters, also heading from Libya to Italy, but they were rejected.

Alexiou argued that any effort to tow the overcrowded trawler or force hundreds of unwilling people onto nearby ships would have been highly hazardous.

“When you … try forcibly to tie up to it or to attach a mooring rope, you will have a disturbance, and the people will surge — which, unfortunately is what happened in the end,” Alexiou told state-run ERT TV. “You will have caused the accident.”

Alexiou also said that, after accepting food from a merchant ship, the trawler’s passengers rejected a rope bringing more from a second merchant ship “because they thought the whole process was a way for us to take them to Greece.”

Experts said maritime law obligated Greek authorities to attempt a rescue regardless.

They definitely “had a duty to start rescue procedures” given the condition of the vessel, said Professor Erik Røsæg of the University of Oslo’s Institute of Private Law. He said a refusal of assistance can be overruled if deemed unreasonable, as it appeared to have been on Wednesday.

Flavio Di Giacomo of the Mediterranean office of the U.N. migration agency IOM tweeted that all migrant boats should be considered dangerous and rescued immediately because “even when they appear to have no problems, in a few minutes they can sink.”

Rescuers pulled 104 survivors from the water and later recovered 78 bodies but have not located any more since late Wednesday. The Greek coast guard said the search-and-rescue operation would continue beyond the standard 72 hours.

The U.N.’s migration and refugee agencies issued a joint statement calling timely maritime search and rescues “a legal and humanitarian imperative” and calling for “urgent and decisive action to prevent further deaths at sea.”

A group of nongovernmental organizations including Amnesty and Doctors Without Borders said the EU should “stop seeing solutions solely in the dismantling” of smuggling networks.

“We urge the EU and member states to set up proactive, state-led search-and-rescue operations in the Mediterranean Sea,” the NGOs said in a joint statement.

“The Greek government had specific responsibilities toward every passenger on the vessel, which was clearly in distress,” Adriana Tidona of Amnesty International said. “This is a tragedy of unimaginable proportions, all the more so because it was entirely preventable.”

Pope Francis, who was discharged from a Rome hospital today nine days after undergoing abdominal surgery, urged European governments to do more to protect people who risk their lives while attempting to find better ones.

“We must do everything possible so that migrants fleeing war and poverty do not meet death while seeking a future of hope,” a tweet from Francis’ Twitter account said.

U.N. Secretary-General Antonio Guterres was more pointed.

“This is a European problem. I think it’s time for Europe to be able, in solidarity, to define an effective migration policy for these kinds of situations not to happen again,” Guterres said during a news conference at U.N. headquarters in New York late Thursday.

Greece and other southern EU nations that typically are the first destinations for Europe-bound asylum-seekers traveling by sea have toughened border protection measures in recent years, extending walls and intensifying maritime patrols.

The EU’s executive commission says the 27-nation bloc is close to an agreement on how member countries can share responsibility in caring for migrants and refugees who undertake the dangerous journey across the Mediterranean.

A judicial investigation is also underway into the causes of the sinking. Greek officials say the vessel capsized minutes after it lost power, speculating that panic among the passengers may have caused the boat to list and roll over.

Greece heads to a general election on June 25 and protests in Athens turned violent late Thursday and led to 21 arrests.

Most of the survivors were being moved today from a storage hangar at the southern port of Kalamata, where relatives also gathered to look for loved ones, to migrant shelters near Athens.

Nine people — all men from Egypt, ranging in age from 20 to 40 — were arrested and detained and charged today of people smuggling and participating in a criminal enterprise. Twenty-seven of the survivors remain hospitalized, health officials said. The smuggling suspects are due to appear in court Monday.

The IOM has estimated the boat carried as many as 750 people. The survivors were all boys and men from Egypt, Pakistan, Syria and the Palestinian territories. Alexiou, citing survivor accounts, said passengers in the hold of the fishing boat included woman and children but that the number of missing, believed to be in the hundreds, still remained unclear.

Officials at a state-run morgue outside Athens photographed the faces of the victims and gathered DNA samples to start the identification process.

The Netherlands-based International Commission on Missing Persons today offered to send teams to Athens to help with the process, warning that this would require an international effort as people from many countries were on the trawler.

And Egypt’s Foreign Ministry said its embassy in Athens is “following up” with the Greek authorities in their search for the missing migrants.

Frances D’Emilio in Rome, Renata Brito in Barcelona, Sarah El Deeb in Beirut, Lebanon, and Costas Kantouris in Thessaloniki, Greece, contributed to this story.