A total of 103 bodies have been recovered and more have been found inside the wreck, coast guards say.
Passengers reportedly threw themselves into the sea when a fire broke out on board. More than 150 of the migrants have been rescued.
Most of those on board were from Eritrea and Somalia, said the UN.
The boat was believed to have been carrying up to 500 people at the time and some 200 of them are unaccounted for.