The victims had suffocated. Navy spokesman Costantino Fantasia said the boat was carrying about 300 migrants when it ran into difficulty off the coast of Libya, Reuters reported on Saturday.
He said the dead were "presumably in the hold" but details were not clear because the rescue mission was still going on. Fantasia said the boat carrying the migrants had not capsized.
The naval vessel Cigala Fulgosi was leading the rescue mission and had saved many other people, he said.
Last Tuesday, up to 50 migrants went missing when a large rubber dinghy sank in the Mediterranean Sea and 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.