The bodies of at least 50 people have been found in the hold of a boat carrying migrants which was intercepted off the coast of Libya, media quoted the Italian coast guard as saying on Wednesday.
The rescue operation was carried out by a Swedish vessel, Poseidon, working with the EU's Frontex border agency, BBC reported.
Thousands of migrants have died and thousands more have been rescued after setting sail from Libya recently.
Wednesday's rescue operation was one of 10 such missions currently taking place in the waters off Libya, the Italian coast guard spokeswoman said.
Smugglers based in Libya were believed to be taking advantage of calmer seas to send more boat loads of migrants towards European shores.
European officials have described the plight of migrants, almost 250,000 of whom have crossed by boat to the continent this year, as "beyond urgent".
So far this year, more than 2,000 migrants have died trying to cross the sea to Europe, the UN said.