
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, spokeswoman for the UN refugee agency UNHCR.
She 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.
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 spokesman 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.