ROME (AFP) - As many as 700 migrants were feared drowned Sunday after their packed boat capsized off Libya in what was described as the deadliest such disaster to date in the Mediterranean.

The UN refugee agency (UNHCR) and Italy's coastguard said only 28 people had survived the wreck. Their testimonies suggested there had been about 700 people on board the 20-metre fishing boat, officials said.

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

The European Union announced an emergency meeting of foreign and interior ministers to discuss what Amnesty International blasted as an avoidable "man-made tragedy".

"As long as countries of origin and transit do not take action to prevent these desperate trips, people will continue to put their lives at risk," the European Commission said in a statement. The statement said the meeting would be part of the EU's approach to try to tackle the problem by working with third countries, but did not give any timetable.

French President Francois Hollande called earlier Sunday for such a meeting, saying Europe "must act" against the growing catalogue of mass drownings of migrants attempting to reach its shores, and calling for closer surveillance of the routes used by people smugglers.

Coastal authorities in Italy and Malta picked up a distress signal from the stricken vessel around midnight local time on Saturday, when it was about 126 km off the Libyan coast and 177 km south of the Italian island of Lampedusa.

The Italian coastguard instructed a nearby merchant ship to go to the scene 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.

A total of 17 boats scoured the area for survivors on Sunday but only 24 bodies had been recovered so far, the coastguard said.

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 most serious 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 fishing boat they were crammed onto caught fire within sight of the coast of Lampedusa. That horrific tragedy was described at the time as a wake-up call to the world but 18 months later there is no sign of a let-up in the numbers attempting the perilous crossing in search of a better life in Europe.

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 at least match in 2015.

Avoidable deaths?

Pope Francis on Sunday led calls for European Union leaders to act to stem the loss of life. "These are men and women like us, brothers seeking a better life," he said in his weekly address to the Roman Catholic faithful in St Peter's square, urging 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." "These latest deaths at sea come as a shock, but not a surprise." 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 (S$13.7 million) a month amid divisions over whether the mission was unintentionally encouraging migrants to attempt the crossing.

Mare Nostrum has been partially replaced by a much smaller EU-run operation called Triton which has a fraction of the assets and specialist manpower previously deployed by Italy.

Prime Minister Matteo Renzi was expected to make a statement on the tragedy later Sunday following an emergency meeting with ministers involved in managing the migrant crisis.

The migrants seeking to reach Europe with the help of people smugglers are generally fleeing conflict or persecution in places such as Eritrea, Afghanistan and Syria, or poverty and hunger in sub-Saharan Africa and South Asia.