Prosecutors are building a case against the men blamed for the Mediterranean's worst migrant disaster as European Union leaders mull a new plan to stop the smugglers.

The United Nations refugee agency said it believes more than 800 people drowned when a boat packed with migrants trying to reach Europe sank on Saturday.

Officials boarded the rescue ship that took the survivors to Sicily and arrested the Tunisian captain and a Syrian crew member of the ship that capsized.

They are accused of illegal immigration charges and the captain was also accused of reckless homicide.

A spokesman for the Geneva-based UN High Commissioner for Refugees said it had finished interviewing most of the 28 survivors.

Adrian Edwards said the UNHCR believes more than 800 died, "making this the deadliest incident in the Mediterranean that we have recorded". Only 24 bodies were recovered.

The survivors were taken to a migrant holding centre in Catania and were "very tired, very shocked, silent", according to Flavio Di Giacomo of the International Organisation of Migration.

Mr Edwards said about 350 of those aboard were believed to have been Eritreans. Others included people from Syria, Somalia, Sierra Leone, Mali, Senegal, Gambia, Ivory Coast, Ethiopia and Bangladesh.

He added that about 1,300 migrants have drowned in the Mediterranean in April, taking the year's death toll to at least 1,776.

The coastguard said it saved 638 migrants in six different rescue operations on Monday alone. Other rescue operations were taking place today, including one south of Calabria in which two merchant ships were asked to lend a hand.

The weekend deaths have jolted the European Union into taking action, with Italy demanding that it not be left alone to shoulder the burden of rescues and that the EU focus on preventing the boats from leaving Libya in the first place.

Ahead of an emergency EU summit on Thursday, foreign and interior ministers approved a 10-point plan at a meeting in Luxembourg that calls for the beefing-up and expansion of the EU border patrol mission, and a "systematic effort to capture and destroy" smugglers' boats.

It calls for closer law enforcement coordination to trace smugglers' funding, which prosecutors have said often evades traditional bank transfers in favour of informal Arab hawala networks, in which migrants' relatives in Europe pay local brokers for each leg of the journey.

Italian premier Matteo Renzi said: "We are facing an organised criminal activity that is making lots of money, but above all ruining many lives."

At a joint news conference with Malta's prime minister, Joseph Muscat, he compared their activity to that of slave traders of centuries past - "unscrupulous men who traded human lives".

Italy launched a robust and expensive search-and-rescue mission in 2013 after 366 migrants drowned off the island of Lampedusa. The politically unpopular Mare Nostrum operation ended last year, and the EU's Frontex border patrol mission took charge, but its limited mandate and resources have prevented it from being effective in saving lives.

The EU plan also calls for member states to ensure all migrants are fingerprinted. As it is, many migrants pass through Italy without being fingerprinted or applying for asylum here, preferring to pay smugglers to get them to northern Europe where they apply for asylum and have better job opportunities.

Officials in Sicily said two actions are suspected to have caused the migrant boat to capsize.

Catania prosecutors said the captain, 27-year-old Tunisian Mohammed Ali Malek, mistakenly rammed his boat into the merchant ship that had come to its rescue, and the migrants then shifted position on the boat, which was already off balance due to the collision.

Officials stressed that none of the crew aboard the Portuguese-flagged King Jacob is under investigation in the disaster.

A spokesman said the crew members did their job in coming to the rescue of a ship in distress and that their activities "in no way contributed to the deadly event".

The survivors told UNHCR that the wreck was caused when one of the smugglers crashed the boat against the King Jacob, according to spokesman Carlotta Sami.

"The survivors said that the person who was steering the boat, their smuggler, was navigating badly, and he did a bad move that made it crash against the bigger ship," Ms Sami. The ship pitched in the water before finally tipping over and sinking.

Several hundred people were locked on the lower level, hundreds more were inside a second level and hundreds more were on deck, according to prosecutors.

Ms Sami said the Portuguese ship had previous experience with migrant rescues and praised its efforts, and that of other merchant ships.

While the search continues for survivors and bodies, the depth of the sea where the tragedy took place off the Libyan coast makes it unlikely more will be recovered.