The Italian navy rescued nearly 500 boat migrants off the coast of Libya on Monday and recovered the bodies of six people who fell out of a leaking rubber vessel.
Squads on two patrol boats and two helicopters were still searching for survivors and any other victims, the Navy said.
Europe's worst migrant crisis since World War Two is now focused on Italy, at Europe's southern frontier, where some 115,000 people had arrived by the end of August, according to the United Nations refugee agency (UNHCR).
The death toll on the route from North Africa to Italy has jumped to one migrant for every 42 making the crossing, compared to one in every 52 last year, a UNHCR spokesman said last week.
The helicopters used in Monday's rescue are based on the Italian aircraft carrier Garibaldi, the flagship of the European Union's anti people-trafficking mission in the Mediterranean.