With reference to the earlier report on people found dead on a boat off Libyan coast, latest news coming in reveal that about 50 people have been found dead in the hold of a boat carrying migrants intercepted off the coast of Libya, the Italian coastguard says.

About 430 people had been rescued alive from the boat, a spokeswoman said.

The rescue operation was carried out by a Swedish vessel, Poseidon, working with the EU's Frontex border agency.

Thousands of migrants have died and many 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 coastguard spokeswoman said.

It was not immediately clear how the migrants found by the Poseidon had died.

Earlier in August, the Italian navy discovered the bodies of 49 people in the hold of a vessel earlier in August.

Those migrants were thought to have died of asphyxiation. Survivors later testified that smugglers had forced them to remain in the hold.

Smugglers based in Libya are believed to be taking advantage of calmer seas to send more boatloads 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 says.