The European Union proposed doubling the size of its Mediterranean search and rescue operations on Monday, as the first bodies were brought ashore of as many as 900 people feared killed in the deadliest known shipwreck of migrants trying to reach Europe.

Three other rescue operations were underway on Monday to save hundreds more migrants in peril on overloaded vessels making the journey from the north coast of Africato Europe.

The mass deaths have caused shock in Europe, where a decision to scale back naval operations last year seems to have increased the risks for migrants without reducing their numbers.

"The situation in the Mediterranean is dramatic. It cannot continue like this," said European Council President Donald Tusk, calling an extraordinary summit of EU leaders for Thursday to plan how to stop human traffickers and boost rescue efforts.

Malta's Prime Minister Joseph Muscat said as many as 900 people may have died in Sunday's disaster off the coast of Libya when a large boat capsized. That would be the highest death toll in recent times among migrants, who are trafficked in the tens of thousands in rickety vessels across the Mediterranean.

As 27 survivors of the disaster arrived in Italy on a coast guard vessel late on Monday, authorities said the captain of the migrant boat and his deputy had been arrested on suspicion of people smuggling.

EU ministers held a moment of silence at a meeting in Luxembourg to discuss the crisis. The bloc's executive, the European Commission, presented a ten-point plan to address the crisis, which would include doubling the size and the funding of "Triton", an EU naval operation in the Mediterranean.

But even that would leave the operation smaller and less well-funded than an Italian mission abandoned last year due to costs and domestic opposition to sea rescues that could attract even more migrants.

"Slavery"

Italy and Malta were working to rescue another two boats carrying an estimated 400 people off the coast of Libya on Monday. Hundreds of kilometres to the east, coast guardsmen were struggling to save scores of migrants from another vessel destroyed after running aground off the Greek island of Rhodes.

The Greek coast guard said at least three people were killed there. Television pictures showed survivors clinging to floating debris while rescuers pulled them from the waves.

Italy's Prime Minister Matteo Renzi compared the smuggling of migrants across the Mediterranean to the African slave trade of centuries ago. "When we say we are in the presence of slavery, we are not using the word just for effect," he said.

Following an investigation launched after hundreds of migrants drowned near the Italian island of Lampedusa in 2013, prosecutors in Palermo on Monday announced they had arrested 24 suspected traffickers suspected of organising the transport of thousands of Ethiopians and Eritreans to Italy.

However, it was not yet clear whether they had any connection with the latest disaster.

"The investigations are continuing and we will see if there is any responsibility for the latest incidents," Palermo police chief Guido Longo told reporters.

Among those named in the investigation were an Ethiopian and an Eritrean based in Libya who were suspected of being two of the key figures profiting from the so-called "Libyan route". They were not among those arrested.

European officials are struggling to come up with a policy to respond more humanely to an exodus of migrants travelling by sea from Africa and Asia to Europe, without worsening the crisis by encouraging more to leave.

"Search and rescue alone is not a silver bullet," said German Interior Minister Thomas de Maiziere. "If you just organise search and rescue, criminals who get the refugees on board will send more boats."

Nevertheless, Chancellor Angela Merkel said that alongside efforts to fight trafficking, more should be done to save those at sea: "We will do everything to prevent further victims from perishing in the most agonising way on our doorstep."

The vessel overturned and sank off the coast of Libya on Sunday, apparently after passengers rushed to one side to attract attention from a passing merchant ship.

A Bangladeshi survivor told police there had been 950 passengers onboard, many locked into the hold and lower deck. However, Catania Chief Prosecutor Giovanni Salvi, who is conducting a homicide investigation into the case, said the figure needed to be treated with caution.

In the Maltese capital of Valletta, coast guard officers brought ashore 24 corpses found so far. Wearing white protective suits, they carried the victims in body bags off the Italian ship Gregoretti and deposited them in hearses as survivors looked on from the deck.

Twenty-seven survivors rescued so far arrived in the Sicilian port of Catania late on Monday. Another survivor was earlier taken to a Catania hospital by helicopter.

In Greece, more than 90 people were rescued from the boat wrecked off the coast of Rhodes: "We have recovered three bodies so far - that of a man, a woman and a child," a coast guard official said.

Among those calling for more compassion from Europe were the United Nations human rights chief and Pope Francis.

"This is a humanitarian emergency that involves us all," the IOM's Italy Director Federico Soda said, calling for a mission equivalent to the Italian operation to be relaunched immediately.

Libya chaos

If the toll is confirmed in Sunday's tragedy, as many as 1,800 migrants will have died so far trying to cross the Mediterranean since the start of this year. The IOM estimates around 21,000 have made the voyage successfully.

In comparison, by the end of April 2014, fewer than 100 had died out of an estimated 26,000 who crossed.

The number of migrants normally surges in the summer, meaning far greater numbers are likely to attempt the voyage in the coming months. In total last year, 174,000 made the journey successfully and around 3,200 died.

The IOM says hundreds of thousands of people could be planning to attempt the crossing from Libya, now in a lawless state with two competing governments at war with each other and both incapable of policing people-smuggling gangs.

Renzi said a military operation in Libya was not on the table, while Malta's Muscat said theUnited Nations should mandate a force to fight human traffickers in Libya.

Human rights group Amnesty International said Thursday's summit would be a litmus test of Europe's commitment to save lives in the Mediterranean, calling for a robust rescue mission.

Bernard Ryan, professor of migration law at Leicester University, told Reuters: "It's a myth to think there's some other solution".

Last week around 400 migrants were reported to have died attempting to reach Italy from Libya when their boat capsized.

Northern EU countries have so far largely left rescue operations to southern states, such as Italy. According to the IOM, Italian coast guard, navy and commercial ships had rescued 10,000 migrants in the Mediterranean in the past few days.