Rescuers have saved about 3,000 refugees but found more than 50 dead on boats near the coast of Libya, according to the Italian coastguard.
At least 55 bodies were discovered on Wednesday on three overcrowded boats in the Mediterranean Sea, the coastguard said.
Of them, 51 were found in the hold of a wooden boat found drifting precariously off the Libyan coast by the Poseidon, a Swedish ship mobilised under the European Union's rescue mission Triton.
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.
The boat on which rescuers on the Poseidon found the 51 corpses was carrying 439 survivors, Reuters news agency reported.
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.
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The coastguard 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 Organization for Migration.
However, media reports said the refugees had choked to death on gas fumes from the small motor boat.
The influx of people, many of whom are fleeing conflict and poverty, has confronted Europe with its worse refugee crisis since World War II, stirring social and political tensions.
The coastguard in Rome coordinated a total of 10 rescue operations on Wednesday, responding to emergency calls which a spokesperson said all came from boats in difficulty in an area about 50km from the Libyan coast.
Vessels from the Italian coastguard and navy, the Malta-based Migrant Offshore Aid Station, humanitarian agency Doctors Without Borders and the Irish navy all carried out rescues.
The coastguard 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.
Calm weather this week appears to have encouraged the smugglers to get as many people as possible out to sea, knowing that, in most cases, they will be picked up by Italian or international boats and taken to southern Italian ports.
On August 15, the Italian navy discovered the bodies of 49 people asphyxiated in the hold of a people smuggler's boat.
Survivors later testified that the victims had been locked below deck and made to stay there by force.
The Italian coastguard coordinated the rescue of nearly 5,000 people over the weekend and the latest operations will lift to more than 110,000 the number of people to have landed at Italian ports this year.
A further 160,000-plus have arrived in Greece, prompting a crisis for which the European Union is struggling to find a solution.
|The Swedish ship was in the area as part of the EU border agency Frontex's search-and-rescue mission known as Triton [Reuters]