Twenty five African migrants were found dead Monday aboard an overcrowded boat off Italy's Lampedusa island, coast guard officials said.
The 50-foot (15-meter) vessel arrived off the southern island with 271 survivors in the early hours of Monday morning.
The dead men are believed to have suffocated in the engine room at the start of the three-day voyage, the BBC reported.
"Given the state in which the corpses were found, they could have been dead for at least 48 hours," doctor Pietro Bartolo said.
It was understood the men had tried to escape the engine room but those on deck could not make room. The only exit was a 20-inch (50-centimeter) wide door, an official said.
There were signs of a struggle on board and an investigation is under way, officials said.
"From what they told us upon arrival there was no air to breathe, apparently they were so crammed there was nowhere to go," UN High Commission for Refugees spokeswoman Laura Boldrini said, according to AP.
"The survivors are shocked."
People smugglers sell desperate Africans passage to Europe on overcrowded and unsafe boats, Boldrini said.
"You risk it all... It's ferocious," she said.
Some of the migrants on board the boat were reported to be from Somalia, which is now in the grip of famine in addition to civil war.
Tens of thousands of Somalis have fled starvation to a massive refugee camp in neighboring Kenya, AP reported.
Around 20,000 migrants have arrived in Italy by boat in recent months, fleeing conflict in Libya and the Middle East as well as hunger in the Horn of Africa.
Around 250 migrants drowned when their boat capsized near the island in April.