At least 40 migrants died in the hold of an overcrowded boat while trying to cross the Mediterranean, Italy's navy said Saturday.
A rescue operation was underway to try and save hundreds of others on board the vessel, it said.
The victims are thought to have suffocated after inhaling fumes from fuel after the boat took on water in the hold, the captain of the navy ship leading the rescue said on Italian state television.
Commander Massimo Tozzi, speaking from his ship, said that when his men boarded the migrant boat they found the dead in the hold "immersed in water, fuel and human excrement".
Tozzi said his ship, the Cigala Fulgosi, had taken on more than 300 survivors, including women and children.
Last Tuesday, up to 50 migrants went missing when a large rubber dinghy sank in the Mediterranean Sea. Nearly 2,000 have been picked up this week.
The Mediterranean has become the world's most deadly crossing point for migrants.
More than 2,000 people fleeing war and poverty have died this year in attempts to reach Europe by boat, compared with 3,279 deaths during the whole of last year, the International Organization for Migration said last week.