Italian authorities say about 30 bodies have been found in a migrant boat which was stopped between Sicily and the North African coast.
Rescuers made the discovery when they boarded a fishing boat carrying around 590 refugees and migrants to evacuate those in most distress, including two pregnant women.
The immigrants apparently died of asphyxiation, Italian news agencies reported citing the navy and coastguard.
It is not the first time Italian rescuers have found migrants dead on overcrowded boats but never before in such large numbers.
The boat is being towed by the Italian navy and is expected to arrive later today in Pozzallo on the southeast coast of Sicily.
The discovery yesterday underlined the scale of the crisis in the waters of the southern Mediterranean, where hundreds have died making the perilous journey to Europe.
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, which have boosted support for the country's anti-immigration opposition Northern League party.
The figures should soon pass the record of 62,000 people who arrived by sea in 2011, when numbers swelled amid the "Arab Spring" uprisings across North Africa and the Middle East.