Libya has refused to take in a group of 177 migrants stranded on an Italian coastguard boat off a Sicilian port after Rome insisted they would not be allowed to disembark.

Italy's Interior Minister Matteo Salvini threatened earlier this week to return the migrants to the North African country unless other European governments offered to take some of them in.

But Mohamed Siala, foreign minister of the UN-backed Libyan unity government, said that "Libya does not accept this unjust and illegal measure because it already has more than 700,000 migrants" on its territory.

In a statement late Wednesday, he called on the international community "to put pressure on the countries of departure to repatriate their nationals", adding that Libya had only served as a transit point.

The Italian boat "Diciotti" arrived on Monday night off the Sicilian port of Catania.

Plunged into chaos following the fall and killing of longtime dictator Moamer Kadhafi in a 2011 NATO-backed uprising, Libya has become a prime transit point for sub-Saharan African migrants making dangerous clandestine bids to reach Europe.

The country takes in migrants whose boats are intercepted in its waters by the Libyan coastguard, but it has repeatedly rejected those rescued by foreign navies or by humanitarian organisations off its coast.