Liby has long been a major transit route for migrants desperate to reach Europe. (Representational)

More than 80 migrants have gone missing and only four were rescued after their boat capsized off Tunisia on its way from Libya to Italy, Tunisia's coast guard said Thursday.

The Red Crescent and the navy told AFP that three Malians and an Ivorian were rescued off Zarzis in southern Tunisia on Wednesday by the coast guard who had been alerted by local fishermen.

The Ivorian, however, died in hospital and one of the Malians has also been hospitalised in intensive care.

Those rescued had told Tunisian officials that 86 people were on board their inflatable boat when it left from the Libyan town of Zuwara, west of Tripoli, on its way to cross the Mediterranean to Italy.

"About 80 migrants are feared dead. More updates are needed in order to confirm what happened and the actual number of missing," Flavio Di Giacomo, a spokesman for the International Organisation for Migration, tweeted.

Libya, which has been wracked by chaos since the 2011 uprising that killed veteran dictator Moamer Kadhafi, has long been a major transit route for migrants, especially from sub-Saharan Africa, desperate to reach Europe.

On Tuesday night, 44 migrants were killed in an air strike on their detention centre in a Tripoli suburb.