The bodies of at least 50 migrants have been found in the hold of a craft off the Libyan coast, the Italian coast guard said on Wednesday.

The boat was sailing with 439 people on board some 50 km north of the Libyan coast, and received assistance in the early afternoon from Swedish ship Poseidon patrolling the Mediterranean as part of the European Union (EU) rescue mission Triton, a coast guard spokesman told Xinhua.

About 50 bodies were found in the hold of the boat, the official added.

It was the second major incident of the kind in 10 days.

On August 15, the Italian navy had rescued some 312 migrants crammed in a boat off the Libyan coast, and 49 people had been found dead in the hold, apparently killed by the lack of air and fuel fumes.

Survivors of this incident gave their testimony to Italian prosecutors, helping authorities to identify and arrest eight alleged human smugglers, namely four Libyans, three Moroccans, and one Syrian national, who are currently in jail in Catania.

Overall, the naval rescue forces coordinated by the Italian coast guard received 10 emergency calls since Wednesday morning, and all of them coming from migrant craft in difficulty in the waters north of Libya.

Italy's ship Fiorillo saved 112 people aboard an inflatable, and one migrant on board was found dead, the coast guard said.

Another 225 people on an inflatable were rescued by a merchant vessel involved in the operations. At least three migrant craft carrying between 500 and 600 people each received assistance from an Irish naval ship, and vessels from Doctors Without Borders and Migrant Offshore Aid Station humanitarian organisations.

Rescue operations were still underway until Wednesday afternoon, the coast guard added.