Thirty of the 70 migrants on board are feared to have drowned while making a desperate bid to reach Europe via the world's deadliest migration trail.
Six migrants have drowned and 30 were missing off the coast of Tunisia after their boat sank during a bid to reach Europe.
Coast guard units rescued a further 34 passengers after the vessel sank off Zarzis near the Libyan border, Tunisian defence ministry spokesperson Mohamed Zekri said on Thursday.
Survivors had said 70 people had been aboard, including Egyptians, Sudanese and a Moroccan, when the boat set off from Libya headed for European shores, he added.
The survivors were taken to a port in Ben Guerdane, according to Tunisian Red Crescent official Mongi Slim.
Both Tunisia and Libya have served as launchpads for migrants making desperate bids to reach Europe, especially in the chaos in Libya that followed the toppling of late leader Muammar Gaddafi in 2011.
The Central Mediterranean route has become the world's deadliest migration trail, according to humanitarian groups.
Departures surged rapidly in 2021, with almost 55,000 migrants reaching Italy in the first 10 months of the year compared with under 30,000 the previous year, according to Rome.
The Tunisian Forum for Economic and Social Rights says that over the first three quarters of last year, the coast guard intercepted 19,500 migrants during crossing attempts.