The move comes after Spanish navy set sail in an attempt to save migrants stuck in the boat off the coast of Italy.
More than 100 migrants stranded off the Italian coast cannot find a home, as Italy refuses to let them ashore until EU states offer them sanctuary.
Italian coast guard vessel Diciotti docks in the Sicilian port of Pozzallo with 522 migrants and a corpse, UN refugee agency says.
The International Organization for Migration (IOM) has in recent months accelerated a "voluntary returns" programme to repatriate migrants from a number of countries. Nigeria now joins Niger in organising bilateral returns.
About 5,000 migrants were picked up off the Libyan coast by emergency services, Italy's navy, aid groups and private boats on Monday, and rescues were continuing on Tuesday, according to an Italian coastguard spokesperson.
International Organization for Migration spokesperson says the boat was believed to be carrying 110 people. Eight bodies were recovered in the rescue operations, which are hampered by poor conditions.
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