Italian police have arrested two suspected people traffickers among the survivors of a boat that capsized off the coast of Libya, while the UN said 800 people died in the Mediterranean’s worst migrant disaster.

Police have detained a Tunisian man believed to be the captain of the vessel and a Syrian, allegedly a member of the crew, taken from a group of 27 haggard survivors who arrived in the Sicilian port of Catania on Monday evening, Sky News reports.

Prosecutors say the men have will face charges of favouring illegal immigration and that the captain will face charges of reckless multiple homicide.

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The two seamen are being held as part of a probe into a catastrophe that has evoked comparisons with the slave trade and allegations of disregard on the part of European governments.

The European Union has proposed doubling the size of its Mediterranean search and rescue operations, as the first of 800 people confirmed killed in the deadliest known shipwreck of migrants trying to reach Europe were brought ashore.

“We can say that 800 are dead,” United Nations High Commissioner for Refugees spokeswoman Carlotta Sami said in Italy on Tuesday.

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 Africa to Europe.

Hundreds of the victims, including an unknown number of children, will have died in horrific circumstances having been locked in the hold or the middle deck of the 20-metre boat that keeled over in darkness as a Portuguese container ship arrived to answer its distress call.

Italian Prime Minister Matteo Renzi has described the traffickers who packed their human cargo into the boat as akin to 18th-century slave traders.

The UN human rights chief Zeid Ra’ad Al Hussein said the horror at sea had been produced by a “monumental failure of compassion” on the part of European governments, who are now under intense pressure to address the humanitarian crisis on their shores.

At talks in Luxembourg on Monday, EU ministers agreed on a 10-point plan to double the resources available to the bloc’s much-criticised maritime border patrol mission Triton and further measures will be discussed at a summit of EU leaders on Thursday.

The survivors, who hailed from Mali, Gambia, Senegal, Somalia, Eritrea and Bangladesh, were all recovering on Tuesday at holding centres near Catania on Sicily’s eastern coast.

Sunday’s disaster was the worst in a series of migrant shipwrecks that have claimed more than 1700 lives this year and nearly 5000 since the start of 2014.

– with AAP