At least 40 migrants have died in an overcrowded boat in the Mediterranean, the Italian navy has said.

Some 320 others were rescued when the vessel was intercepted off Libya.

The dead were found in the fishing boat’s hold. It is thought they died after inhaling fumes from the engine, the rescue vessel’s captain said.

European officials say the plight of migrants, almost 250,000 of whom have crossed by boat to the continent this year, is “beyond urgent”.

So far this year, more than 2,000 migrants have died trying to cross the sea to Europe, the UN says.