Twenty-five African migrants trying to reach Italy from Libya died in the hold of a rickety boat so packed with people that they could not get out as they struggled to breathe, officials said Monday after the bodies were found below decks.

The 15-metre boat was carrying 296 people, including women and children, Italian coast guard Capt. Antonio Morana said.

Lampedusa, one-third the size of Manhattan, is 110 kilometres from Tunisia and 205 km from Sicily.

Some of them were stowed away in the hold, which also served as an engine room, according to the ANSA news agency. As the air became unbreathable from exhaust fumes, migrants tried to exit but the boat was too packed for those standing above to move aside.

"From what they told us upon arrival, there was no air to breathe. Apparently they were so crammed there was nowhere to go," said Laura Boldrini, spokeswoman in Italy for the UN High Commissioner for Refugees.

At least some of the migrants may have died from asphyxiation, Boldrini said.

Hundreds of migrants fleeing unrest and conflict in Libya and across North Africa are believed to have died since the beginning of the year in desperate journeys across the Mediterranean.

The officials found the most recent corpses — all young men — after boarding the boat just a few kilometres off Lampedusa, a small Italian island closer to North Africa than the Italian mainland. One photo posted on Italian newspapers showed the lifeless body of a man being pulled out of the boat by mask-wearing rescuers.

The bodies were put in green bags and lined up on Lampedusa's shore, before being moved to an immigrant shelter on the island. An autopsy and investigation will take place, according to an official.

Survivors were taken ashore and also moved to the immigrant shelter.

"The survivors are shocked," Boldrini added.

Unscrupulous smugglers pack too many people on unsafe boats, she said. And "this leads to a situation on board where your death is my life, where you risk it all. It's ferocious."

All the victims were believed to be of sub-Saharan origin. According to the survivors, the boat had set sail from Libya two days ago. Boldrini said some 50 Somalis were believed to be on the boat, fleeing famine in the Horn of African region.

Some 20,000 people have arrived in Italy by boat in recent months following unrest in Libya and Tunisia.