Fifty-one refugees were found dead in the hold of a boat off the coast of Libya on Wednesday during a rescue operation that saved 439 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 representative for the Italian coast guard said the Swedish ship Poseidon, working with the European Union's border control agency Frontex, went on Wednesday to help a boat in difficulty and found the bodies.
"We had to cut up the deck of the wooden boat to get to (the bodies) in a safe way," Swedish coast guard spokesman Mattias Lindholm told The Associated Press.
He said the victims probably died of asphyxiation. The corpses were placed in body bags to make sure they didn't come into contact with the survivors.
More than 100,000 refugees 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 from the Libya coast, the representative said.
An Italian coast guard vessel saved 113 people from a partly deflated rubber boat, but one passenger, who was already fighting for their life, died soon after.
The Poseidon 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 representative said.