The 73 migrants were picked up from a packed rubber dinghy drifting about 50 miles off Libya on Wednesday night.
AP investigation finds that African migrants are starved, raped and tortured in the Yemeni town of Ras Al Ara, which profits from human smuggling.
The boat sank 10 kilometres from the coastline near the city of Sfax, a member of the coastguard said. Rescue operations are continuing.
The International Organisation for Migration says that since the start of this year at least 560 migrants have died trying to get to Spain.
Every year sub-Saharan African migrants living illegally in Morocco try to enter Europe either by climbing over their border fences or swimming along their coastlines.
Under a deal reached with the UNHCR just hours before the Israeli premier's announcement, over 16,000 migrants and refugees would be sent to various Western countries, while much of those remaining in Israel would be granted official status.
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