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.

In this image from TV, migrants crowd at the rail aboard an Italian navy vessel as it cruises towards Italian port of Messina, Saturday April 18, 2015. UNHCR says Sunday, April 19, 2015, the search and recovery rescue operation is under way after a boat carrying migrants overturned north of Libya.

In this image from TV, migrants crowd at the rail aboard an Italian navy vessel as it cruises towards Italian port of Messina, Saturday April 18, 2015. UNHCR says Sunday, April 19, 2015, the search and recovery rescue operation is under way after a boat carrying migrants overturned north of Libya. Image by (AP Photo/APTV)

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.

(Press Association)