The International Organization for Migration on Thursday said that a boat carrying 86 migrants from Libya sank on Wednesday in the Mediterranean Sea off the Tunisian city of Zarzis.
Only three passengers on board survived the incident.
Lorena Lando, the head of the IOM in Tunisia, said that Tunisian fishermen pulled out four men from the sinking boat when they stumbled upon it but were unable to find any other passengers.
One of the four men rescued died overnight while the other three remained hospitalised.
The deaths came just a day after an airstrike on a Libyan detention centre in a suburb of the Libyan capital of Tripoli killed at least 44 migrants and wounded 80 others.
The Tripoli-based government said in a statement that dozens of people had been killed and wounded in an airstrike blamed on Khalifa Haftar.
It is the highest publicly reported toll of an airstrike or shelling since eastern forces loyal to Khalifa Haftar launched an offensive with ground troops and aircraft three months ago to take the capital held by the United Nations-backed government.
The conflict is part of chaos that has taken place in Libya since the NATO-backed overthrow of Muammar Gaddafi in 2011.