A smuggler's boat crammed with hundreds of people overturned off Libya's coast as rescuers approached, causing what is feared to be the Mediterranean's deadliest known migrant tragedy.
Italian prosecutors said a Bangladeshi survivor told them 950 people were aboard, including hundreds who had been locked in the hold by smugglers.
Italian premier Matteo Renzi said authorities were "not in a position to confirm or verify" the death toll.
Eighteen ships joined the rescue effort, but only 28 survivors and 24 bodies were pulled from the water by nightfall, Mr Renzi said.
The small numbers make more sense if hundreds of people were locked in the hold, because with so much weight down below, "surely the boat would have sunk," said General Antonino Iraso, of the Italian Border Police, which has deployed boats in the operation.
Prosecutor Giovanni Salvi said a survivor from Bangladesh described the situation on the fishing boat to prosecutors who interviewed him in a hospital. The man said about 300 people were in the hold when the fishing boat overturned, and that about 200 women and dozens of children also were on board.
Mr Salvi stressed that there was no confirmation yet of the man's account and that the investigation was ongoing.
Mr Iraso said the sea in the area is too deep for divers, suggesting that the final toll may never be known.
"How can it be that we daily are witnessing a tragedy?" asked Italian Premier Matteo Renzi, who met with his top ministers ahead of tomorrow's European Union meeting in Luxembourg, where foreign ministers have added the issue of migrants to their agenda.
So far this year, 35,000 asylum seekers and migrants have reached Europe and more than 900 are known to have died trying.
On the back of today's tragedy, demands for decisive action were being made by France, Spain, Germany and Britain.
"Europe can do more and Europe must do more," said Martin Schulz, president of the European Parliament. "It is a shame and a confession of failure how many countries run away from responsibility and how little money we provide for rescue missions."
Europe must mobilise "more ships, more overflights by aircraft," French President Francois Hollande told French TV Canal +. "Words won't do anymore," Spain's Prime Minister, Mariano Rajoy, told a political rally.
Mr Renzi said he too wants action, but he rejected calls by some Italian lawmakers for a naval blockade. That would only "wind up helping the smugglers" since military ships would be there to rescue any migrants, and they wouldn't be able to return passengers to chaos and violence in Libya.
Meanwhile rescuers were "checking who is alive and who is dead" in an area littered with debris and oil from the capsized ship. Maltese Prime Minister Joseph Muscat, whose island nation joined the effort, said only 50 survived, and called it the "biggest human tragedy of the last few years".
The 20m (66-foot) vessel may have overturned because migrants rushed to one side of the craft late Saturday night when they saw an approaching Portuguese-flagged container ship, the King Jacob, which was sent to the area by Italy's Coast Guard. The ship's crew "immediately deployed rescue boats, gangway, nets and life rings," a spokesman for its owner said.
"Since the waters of the Mediterranean Sea are not too cold at the moment, the authorities hope to find more survivors," said International Organization for Migration spokesman Joel Millman.
United Nations refugee agency spokeswoman Carlotta Sami tweeted that according to one survivor, the boat had set out with 700 migrants aboard. When it overturned, "the people ended up in the water, with the boat on top of them," Ms Sami told Italian state TV.
"There are fears there could be hundreds of dead," Pope Francis said in St Peter's Square, lending his moral authority to the political calls for action by urging "the international community to act decisively and promptly, to prevent similar tragedies from occurring again".