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 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.
The migrant crisis has exploded upon Europe's southern Mediterranean shores this year and moved inland towards countries like Macedonia, which is trying to control the flow of migrants.
The Christian Science Monitor reported earlier this week on the effects migrants are having on both countries.
The turmoil at the Macedonian border coincides with reports from Italy's coast guard about the rescue of some 4,400 migrants in a single day, a record-setting number, as smugglers took advantage of ideal sea conditions off Libya to launch a fleet of overcrowded, unseaworthy boats, The Associated Press reports.
The coast guard on Sunday said it carried out 22 rescue operations the previous day, with help from Norwegian and Irish vessels, which provided help to motorized rubber dinghies and fishing boats, all brimming with people trying to reach Europe.
So far this year, some 110,000 migrants have been rescued off Libya and brought to southern Italian ports, the AP reports.