At least 40 migrants have died on a boat in the southern Mediterranean, Italian authorities have reported.

The Italian navy wrote on Twitter on Saturday that it was involved in a rescue operation to try and save the other migrants on board.

It was initially reported that the boat had sunk, but an Italian navy spokesman later told the Reuters news agency that the boat had not capsized.

Those killed were believed to have been stuck in the hold of the boat, spokesman Costantino Fantasia said.

The navy said 300 people have been rescued during the operation so far.

Italy's ANSA news agency reported that the boat was located about 33km off Libya's coast and was holding about 400 people on board.

The naval vessel Cigala Fulgosi was leading the rescue mission.

The Mediterranean has become the world's most deadly crossing point for migrants.

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.

Around 200 migrants were presumed killed earlier this month off the coast of Libya when their boat capsized