A major rescue operation is under way in the Mediterranean after as many as 700 migrants are feared to have drowned just outside Libyan waters, in what could prove to be the worst disaster yet involving migrants being smuggled to Europe.

Italian coastguards have retrieved 49 survivors so far and about 20 bodies, according to the interior ministry, after the boat went down overnight about 60 miles (96km) off the Libyan coast and 120 miles (193km) south of the Italian island of Lampedusa.