The Italian coast guard found 25 dead bodies in the engine room of a refugee boat fleeing Libya with 271 people crammed on board that arrived on the holiday island of Lampedusa Monday, port officials said.
"It's 25 bodies of men, presumed to be from sub-Saharan Africa," Antonio Morana, the commander of Lampedusa port, said on news channel SkyTG24.
"The cause of death is still being investigated," he said.
Morana said refugees on board had said they left Libya three days ago.
Prosecutors, who have opened an inquiry, said the men appear to have choked to death in the heavily overcrowded boat, which was intercepted by coast guards in Italian waters late Sunday.
"We will carry out an autopsy to find the precise cause of death even though initial checks show that it was apparently due to asphyxiation," said a local prosecutor, Renato Di Natale.
The refugees did not mention the dead bodies and the macabre discovery happened only when officers made a final inspection of the boat after transferring the refugees onto a coast guard vessel one mile from the coast.
Coast guards had been about to abandon the broken-down refugee boat.
The refugees who died were in an engine room accessible only through a 50-centimetre (20-inch) wide trap door, and were removed by firemen.
The 271 refugees found alive included 36 women and 21 children.
Thousands of refugees fleeing Libya, mostly migrant workers from other parts of Africa, have arrived on tiny Lampedusa in recent weeks.
Hundreds drowned in a series of accidents in stormy seas. The vessels used for crossings from Libya are often rickety fishing boats.
The journey from North Africa to Italian shores usually takes around two days.
Lampedusa, which has a surface area of just 20 square kilometres, is Italy's southernmost point and is closer to North Africa than to the Italian mainland.
It is now the biggest gateway for illegal immigration into the European Union following the arrival of tens of thousands of Tunisian migrants this year.
The almost daily arrivals of refugees from Libya and migrants from Tunisia seen in spring have slowed in recent weeks, with only occasional landings.
Scenes of desperation and chaos seen earlier this year have hit the pristine island's tourism industry but there are already many holidaymakers on the beaches for the high season and there is a summer film festival underway.