Rome - EU ministers headed on Monday into crisis talks under pressure to confront people-smuggling gangs, with more than 700 people feared dead in what may be the Mediterranean's deadliest migrant disaster to date.
Italy's coastguard said only 28 people had survived a shipwreck off war-torn Libya, which was seen by many as an avoidable tragedy.
"It seems we are looking at the worst massacre ever seen in the Mediterranean," said Carlotta Sami, spokesperson for the UN refugee agency UNHCR.
She said survivors' testimonies suggested there had been around 700 people on board the 20m fishing boat when it keeled over in darkness overnight.
A Bangladeshi survivor who was taken to hospital by helicopter 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 Italian city of Catania.
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 were set to discuss the immigration disaster at a previously scheduled meeting in Luxembourg later on Monday. EU president Donald Tusk was considering holding a special summit on the crisis, his spokesperson told AFP.
Tusk said on Twitter that he "will continue talks w/ EU leaders, Commission & EEAS [EU diplomatic service] on how to alleviate situation".
Member states Spain, Greece, Germany and France urged immediate action, with Italian Prime Minister Matteo Renzi leading calls for a summit by the end of the week.
The latest disaster comes after a week in which two other migrant shipwrecks left an estimated 450 people dead, with increasing boatloads coming from Libya as the North African country falls deeper into chaos.
If the worst fears about Sunday's tragedy are confirmed, it would 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.
Rights groups including Amnesty International are 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 $9.7m a month amid divisions over whether the mission was unintentionally encouraging migrants to attempt the crossing.
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Mare Nostrum has been replaced by a much smaller EU-run operation called Triton.
UN Secretary General Ban Ki-moon said governments worldwide should show solidarity and take in more refugees, adding he was "shocked and deeply saddened" by reports of the latest shipwreck.
Governments must not only improve rescue at sea but also "ensure the right to asylum of the growing number of people worldwide fleeing war who need refuge and safe haven", Ban added.
Amnesty International described Sunday's disaster as a predictable "man-made tragedy".
The fishing boat capsized most likely as a result of terrified passengers stampeding to one side in their desperation to get off, the UNHCR's Sami said, after coastal authorities in Italy and Malta picked up a distress signal around midnight on Saturday, when it was still in Libyan waters.
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 proper 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."