Rescuers saved about 3,000 migrants but found more than 50 dead on boats near the coast of Libya on Wednesday, the Italian coast guard said.
Tens of thousands of people, mainly from Africa and the Middle East, have put to sea this year in the hope of reaching Europe, often dangerously packed into small vessels that were never designed to cross the Mediterranean.
Rescuers on the Swedish ship Poseidon, mobilised under the European Union’s rescue mission Triton, found 51 corpses in the hold of one boat which was also carrying 439 survivors.
Three women were found dead on a rubber boat carrying a further 120 people. One person rescued along with more than 100 others on another boat died shortly afterwards.
The coast guard did not say what caused the deaths, which add to a toll already thought to have exceeded 2,300 so far this year, according to the International Organisation for Migration.
The influx of migrants, many of whom are fleeing conflict and poverty, has confronted Europe with its worse refugee crisis since World War Two, stirring social and political tensions.
A spokeswoman for the Italian coast guard said earlier on Wednesday that the boat carrying the migrant who died shortly after the rescue had already partially deflated by the time the emergency services arrived.
The coast guard in Rome coordinated a total of 10 rescue operations Wednesday, responding to emergency calls which the spokeswoman said all came from boats in difficulty in an area around 30 miles (50 km) from the Libyan coast.
Vessels from the Italian coast guard and navy, the Malta-based Migrant Offshore Aid Station, humanitarian agency Doctors Without Borders, and the Irish navy all carried out rescues.
The coast guard said a merchant ship which had gone to the rescue of 225 people was heading for the Greek island of Crete, where the survivors would disembark.