As many as 30 corpses were found on a boat packed with migrants off the coast of Sicily, Italy's navy said on Monday after rescuing thousands of people trying to cross from North Africa over the weekend.
The dead are thought to have either suffocated on the overcrowded fishing vessel or drowned, the navy said.
The discovery on Sunday underlined the scale of the crisis in the southern Mediterranean, where hundreds have died in the past year making the journey to Europe, and tens of thousands more have been plucked from rickety boats.
More than 5,000 people were rescued this weekend, adding to the 50,000 migrants who have reached Italy from North Africa so far this year, many fleeing war and forced conscription.
Italy has called on its European Union partners to do more to help manage the near-daily arrivals, a phenomenon that has boosted voter support for the anti-immigration Northern League party in a country struggling to emerge from recession.
At the current rate, the figures should soon pass the record of 62,000 people who arrived by sea in 2011, the year of the Arab Spring turmoil across North Africa and the Middle East.
An Italian ship from the navy's migrant rescue mission Mare Nostrum, or "our sea," was due to arrive at the Sicilian port of Pozzallo on Tuesday morning towing the fishing boat and carrying 566 survivors.
The navy said thousands of others rescued this weekend would arrive in other ports on Monday and Tuesday.
Prime Minister Matteo Renzi said last week the EU should take responsibility for rescuing migrants by investing more in regional border-control agency Frontex.
Renzi has also urged the United Nations to intervene in Libya, where traffickers charge migrants more than $1,000 each for the risky passage.
European Home Affairs Commissioner Cecilia Malmstrom said the commission was making available to Italy €4-million ($5.8-million) of emergency funding and was looking at ways to contribute more.
"This new death toll clearly illustrates that smugglers and criminals have no respect for human lives and we must urgently increase our efforts to fight their deadly activities," Malmstrom said.
Northern League leader Matteo Salvini, whose party says Mare Nostrum is a burden on taxpayers and encourages immigration, attacked Renzi and Interior Minister Angelino Alfano in a Facebook post:
"Thirty more deaths on the consciences of those who defend 'Their Seas.' Stop the departures, help them at home, now! Renzi and Alfano have blood on their shirts, don't they?"
Mare Nostrum is Europe's biggest search and rescue mission and costs around €9-million a month.