About 20 African migrants were found dead on Thursday after their boat, which was trying to reach Europe, sank in the Mediterranean Sea, Tunisian authorities said.

Five survivors were rescued and the Tunisian navy is searching for up to 20 others still believed missing.

Tunisian coast guard boats and local fishermen found the bodies off the coastal city of Sfax in central Tunisia, Defense Ministry spokesman Mohamed Ben Zekri told The Associated Press.

According to the survivors, the migrant smuggling boat was carrying about 40 or 50 people heading toward Italy, Ben Zekri said.

"The boat was overloaded and in poor condition, and faced strong winds on Thursday morning that may have contributed to the sinking," said National Guard spokesman Ali Ayari.

"It was carrying migrants from sub-Saharan Africa," he told The AP.

Tunisian navy units were on the scene to search for any more survivors.

Tunisian authorities say they have intercepted several migrant boats recently but that the number of attempts has been growing, notably between the Sfax region and the Italian island of Lampedusa.

Migrant boats frequently leave from the coast of Tunisia and neighbouring Libya, carrying people from across Africa, including a growing number of Tunisians fleeing prolonged economic difficulties in their country.

Tunisians have made up the vast majority of migrants arriving in Italy this year, despite efforts by Rome to negotiate with Tunis to put a stop to the crossings.

Of the 34,001 migrants who had arrived in Italy so far this year, 12,847 were Tunisian, or 38 per cent. Bangladeshis were the next biggest group, followed by those from Ivory Coast, Algeria, Pakistan and Egypt.

Earlier this month, at least 120 Europe-bound migrants, including eight women and 28 children, were intercepted in the Mediterranean Sea by Libya’s coast guard.

The International Orga­nisation for Migration (IOM) had said that a vessel carrying the migrants was stopped off the coast of the North African country and that the migrants were returned to Libya.

Safa Msehli, an IOM spokesperson in Libya, had said that 126 migrants from the vessel were taken to detention centres inside Libya.

In November, at least 74 migrants died in a “devastating” shipwreck off the Libyan coast.