Some 50 migrants were found dead in the hold of a boat off the coast of Libya on Wednesday during a rescue operation which saved 430 other people, the Italian coast guard said.
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 onto small boats that were never designed to cross the Mediterranean.
A spokeswoman for the Italian coast guard said the Swedish ship Poseidon, working with the European Union's border control agency Frontex, had gone on Wednesday to help a boat in difficulty and had found the bodies.
It was not immediately clear how the migrants died.
More than 100,000 migrants have reached Italy so far this year, but at least 2,300 are believed to have died while trying to make the crossing.
Emergency services received 10 calls for help from various boats in difficulty on Wednesday, all in an area around 30 miles (50 km) from the Libya coast, the spokeswoman said.
An Italian coast guard vessel saved 113 people from a partially-deflated rubber boat, but one passenger, who was already fighting for their life, died soon after.
The Poseidon had earlier saved 130 people from another rubber boat and a merchant ship picked up 225 people in a separate operation.
An Irish coast guard ship is in the process of rescuing up to 500 people from another boat, the spokeswoman said. Reuters