A smuggler's boat crammed with hundreds of people overturned off Libya's coast as rescuers approached, causing what could be the Mediterranean's deadliest known migrant tragedy and intensifying pressure on the European Union to finally meet demands for decisive action.

Survivor accounts of the number aboard the 20-metre vessel varied, with the Italian Coast Guard saying that the capsized boat had a capacity for "hundreds" of people.

Italian prosecutors said a Bangladeshi survivor flown to Sicily for treatment told them 950 people were on board, including hundreds who had been locked in the hold by smugglers. Earlier, authorities said a survivor told them 700 migrants were aboard.

Italian Prime Minister Matteo Renzi said authorities were "not in a position to confirm or verify" how many were on board when the boat set out from Libya.

Eighteen ships joined the rescue effort, but only 28 survivors and 24 bodies had been pulled from the water by nightfall, Renzi said.

These 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.

The incident happened in an area just off Libyan waters, 193km south of Lampedusa island, according to a report in the Times of Malta's website.

Rescue boats deployed

Al Jazeera's Paul Brennan, reporting from Catania in Sicily, off Italy's southern coast, said while the weather was sunny and clear on Saturday, it later turned overcast and cloudy, which meant that the seas were choppy.

He said reports indicated that the approach of a Portuguese-flagged container ship sent by Italy's coast guard prompted people on board the boat to shift to one side, upsetting the weight distribution and causing it to sink.

The container ship's crew "immediately deployed rescue boats, gangway, nets and life rings", a spokesman for its owner said.

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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", Sami said.

Prosecutor Giovanni Salvi told The Associated Press that the Bangladeshi survivor said about 300 people were locked in the hold by smugglers when the vessel set out. He said some 200 of the boat's passengers were women and several dozen were children.

Salvi stressed that there was no confirmation yet of the man's account and that the investigation was ongoing.

Unified response

Meanwhile, calls by Italy's Renzi for a more unified response from the EU were echoed by France, Spain, Germany and Britain ahead of Monday's EU meeting in Luxembourg, where foreign ministers scrambled to add stopping the smugglers to their agenda.

"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 on Sunday. "Words won't do anymore," Spain's prime minister, Mariano Rajoy, told a political rally.

Meanwhile, Renzi rejected calls by some Italian legislators for a naval blockade. That would only "wind up helping the smugglers" since military ships would be there to rescue any migrants, and they would not be able to return passengers to the violence in Libya.

Maltese Prime Minister Joseph Muscat, whose nation joined the search effort, called it the "biggest human tragedy of the last few years".

Since the start of 2014, Italy has rescued nearly 200,000 people at sea, including 11,000 arrivals in the eight days up to Saturday.

At least 900 people have died trying to reach Europe this year, before this latest sinking.

Source: Al Jazeera and agencies