Italian police have arrested two suspected people traffickers among the survivors of the migrant boat that capsized off Libya on Sunday, as the UN said 800 people were killed in the Mediterranean’s worst migrant disaster.

They said they had detained a Tunisian man believed to be the captain of the vessel and a Syrian allegedly a member of the ship's crew, taken from a group of 27 haggard survivors who arrived in the Sicilian port of Catania on Monday evening. Both face charges of people trafficking.

Under-fire EU ministers meanwhile agreed on a 10-point plan to double the resources available to the current EU border surveillance mission Triton, as the UN's refugee agency and the International Organization for Migration recounted what those onboard had witnessed.

"We can say that 800 are dead," said Carlotta Sami, spokeswoman for the United Nations High Commissioner for Refugees in Italy on Tuesday, citing the survivors' accounts of the deadly crossing.

Those who escaped with their lives described to officials the moment the 20-metre (70-foot) trawler carrying them capsized after a Portuguese merchant ship approached the vessel, causing a stampede.

"There were a little over 800 people on board, including children aged between 10 and 12. There were Syrians, about 150 Eritreans, Somalians... They had left Tripoli at about 8 am on Saturday," Sami said.

The survivors hailed from Mali, Gambia, Senegal, Somalia, Eritrea and Bangladesh, she added, and all had been taken to nearby holding centres.

One other survivor was taken to hospital in Catania, on Sicily's east coast.


'No more excuses'

EU foreign affairs chief Federica Mogherini had unveiled plans earlier Monday to tackle the growing migrant crisis after telling member states they had "no more excuses" not to act.

Even as EU foreign and interior ministers met in Luxembourg to discuss ways to stem the flood of people trying to reach Europe, the International Organization for Migration said it had received a distress call from another boat -- but cautioned against concluding this was another disaster in the making.

Italian Premier Matteo Renzi said separately that Italy's coast guard had asked merchant shipping to come to the aid of two boats off the Libyan coast with up to 450 migrants on board after they sought help.

Police in Greece meanwhile reported three people killed, including a child, after a boat coming from Turkey sank off the island of Rhodes.

Dramatic footage showed people trying to reach survivors huddled on a piece of wreckage as they were being swept towards rocks.

Ninety-three people were rescued alive, police said.

Europe's southern shores have been swamped over the past two weeks with migrants fleeing war and hardship, mostly via conflict-wracked Libya.

More than 11,000 migrants have been rescued by Italian authorities since the middle of last week alone and current trends suggest last year's total of 170,000 landing in Italy is likely to be exceeded in 2015.

Unveiling the 10-point action plan, Mogherini said the EU had to live up to its humanitarian values and commitments towards migrants, she said, adding: "To send them back is another way of killing them."

First on the list, ministers agreed the current EU border surveillance mission Triton should be increased to extend its range and capabilities on the bloc's southern flank.

Triton replaced Italy's own Mare Nostrum mission, which Rome scrapped late last year in protest that its EU partners would not share the burden.

The EU will also try to capture or destroy people-smuggling boats and increase cooperation across the board, the European Commission said.

The bloc will also offer a "voluntary pilot project on resettlement, providing a number of places to persons in need of protection", a key but small step forward in spreading the problem.

Up to now, countries relatively untouched by the problem had objected to this form of burden sharing, however small.

Diplomats said there could be 5,000 places available but the commission gave no figure.


Focus on people smugglers

Elsewhere EU president Donald Tusk announced an emergency leaders summit for Thursday to discuss the plan, saying: "We cannot continue like this, we can't accept that hundreds of people die."

Italy's Renzi, whose country bears the brunt of the problem, said Rome was studying the possibility of mounting "targeted interventions" against Libya-based people smugglers.

"Attacks on death rackets, attacks against slave traders (traffickers) are in our thinking," Renzi told a press conference with his Maltese counterpart Joseph Muscat.

Italian and Maltese navy boats meanwhile continued to search for the victims of Sunday's disaster, which brings to an estimated 1,600 the number of migrants who have drowned in the Mediterranean this year.

Only 28 survivors have been found so far, along with 24 bodies, which were taken to Malta.

The deadliest incident prior to Sunday occurred off Malta in September 2014, when an estimated 500 migrants drowned after traffickers deliberately rammed their boat in an attempt to force the passengers onto a smaller vessel.