More than 700 people are feared to have drowned after an overcrowded boat smuggling them to Europe capsized off Libya, officials said Sunday, prompting demands for the European Union to react to the Mediterranean's deadliest migrant disaster to date.

Italy's coastguard, which was coordinating the search for survivors and bodies, said only 28 people had survived a wreck that triggered fresh calls from Pope Francis and others for European leaders to act over what many saw as an avoidable tragedy.

The UN refugee agency (UNHCR) said survivors' testimonies suggested there had been around 700 people on board the 20-metre (70-foot) fishing boat when it keeled over in darkness overnight, officials said.

"It seems we are looking at the worst massacre ever seen in the Mediterranean," UNHCR spokeswoman Carlotta Sami said.

A Bangladeshi survivor who was helicoptered to hospital in Sicily put the numbers on board at 950 and said 200 women and children and nearly 50 children had been among them, according to prosecutors in the city of Catania.

European Council President Donald Tusk was considering holding a special EU summit on the crisis, his spokesman told AFP, amid calls for urgent action from member states such as Spain, Greece, Germany and France.

Tusk said on Twitter that he "will continue talks w/ EU leaders, Commission & EEAS (EU diplomatic service) on how to alleviate situation".

- EU under pressure -

Italy led the calls Sunday for an emergency summit, with Prime Minister Matteo Renzi saying Rome was working "to ensure this meeting can be held by the end of the week".

Spanish Prime Minister Mariano Rajoy said: "We Europeans risk damaging our credibility if we are not able to prevent these tragic situations which are happening every day."

EU foreign ministers are set to discuss the immigration crisis at a meeting in Luxembourg on Monday.

Anger among non-governmental groups was underlined by Amnesty International, which described the latest disaster as a predictable "man-made tragedy".

Coastal authorities in Italy and Malta picked up a distress signal from the stricken vessel around midnight (2200 GMT) on Saturday, when it was still in Libyan waters.

The Italian coastguard instructed a nearby merchant ship to provide assistance and it was when the Portuguese-registered King Jacob arrived at the scene that the fishing boat capsized, most likely as a result of the terrified passengers stampeding to one side in their desperation to get off, the UNHCR's Sami said.

Italian, Maltese and merchant boats scoured the area for survivors but only 24 bodies were recovered. They were due to arrive in Malta on Monday morning while survivors are being taken to Sicily.

Premier Renzi said the coastguard would seek to salvage the boat and ensure any corpses recovered from it were given a decent funeral.

EU foreign policy chief Federica Mogherini, a former Italian foreign minister, called the disaster a stain on the EU's conscience.

"We have said too many times 'never again'. Now is time for the European Union as such to tackle these tragedies without delay."

The UN High Commissioner for Refugees, Antonio Guterres, added: "This disaster confirms how urgent it is to restore a robust rescue-at-sea operation and establish credible legal avenues to reach Europe. Otherwise people seeking safety will continue to perish at sea."

- Worst tragedy yet -

The disaster was the latest in a growing catalogue of mass drownings of migrants attempting to reach Europe on overcrowded, unseaworthy boats run by people smugglers who are able to operate out of Libya with impunity because of the chaos engulfing the north African state.

The deadliest incident prior to Sunday occurred off Malta in September 2014. An estimated 500 migrants drowned in a shipwreck caused by traffickers deliberately ramming the boat in an attempt to force the people on board onto another, smaller vessel.

In October 2013, more than 360 Africans perished when the tiny boat they were crammed onto caught fire within sight of the coast of Lampedusa.

The latest disaster comes after a week in which two other migrant shipwrecks left an estimated 450 people dead.

If the worst fears about Sunday's tragedy are confirmed, it will take the death toll since the start of 2015 to more than 1,600.

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

Pope Francis urged EU leaders to "act decisively and quickly to stop these tragedies from recurring".

Amnesty's John Dalhuisen called Sunday's accident a "man-made tragedy of appalling proportions."

Amnesty is among NGOs calling for the restoration of an Italian navy search-and-rescue operation known as Mare Nostrum which was suspended at the end of last year.

Italy scaled back the mission after failing to persuade its European partners to help meet its operating costs of nine million euros ($9.7 million) a month amid divisions over whether the mission was unintentionally encouraging migrants to attempt the crossing.

Mare Nostrum has been replaced by a much smaller EU-run operation called Triton.
Source: AFP