Officials boarded the rescue ship that took the 27 survivors of the weekend disaster to Sicily and arrested the Tunisian captain and a Syrian crew member of the ship that capsized.

Now prosecutors are building a case against the men blamed for what might be the Mediterranean's worst migrant disaster. Whilst European Union leaders mulled a new plan to stop the smugglers.

They are accused of illegal immigration charges and the captain was also accused of reckless homicide in the deaths of 700-900 migrants.

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.

"They told us that there were 800 on board," he said. "Everyone told us that there were about 800 so we have to say that 800 people have died at this point."

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

An immigrant treats a co traveller lying on a coast in the eastern Aegean island of Rhodes, Greece on Monday, April 20, 2015 .Greek authorities said that at least three people have died, including a child, after a wooden boat carrying at least 83 migrants from the Turkish shore ran around off the island of Rhodes. (AP Photo//Nikolas Nanev)

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".

In this image taken from video from www.rodiaki.gr a rescuer, left back to camera, helps migrants from the wreckage in the eastern Aegean island of Rhodes, Greece on Monday, April 20, 2015 .Greek authorities said that at least three people have died, including a child, after a wooden boat carrying tens migrants ran aground off the island of Rhodes. (www.rodiaki.gr via AP)

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 say 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 Portuguese-flagged 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.

The prosecutors say the death toll is still uncertain, noting that the passengers have spoken of anywhere between 400 and 950 people on board, while the crew of the rescue ship estimated 850.

Only 24 bodies were recovered, while 28 people survived.