The navy said it had launched a search involving three vessels and a Spanish maritime patrol aircraft.
“Nine lifeless bodies were discovered and three survivors identified. An unknown number of people have been rescued by local fishermen”, it said, without giving the number of missing.
The search is ongoing along the coast, AFP journalists saw.
Several locals said the boat was carrying dozens of migrants, but AFP was unable to verify this.
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Senegal’s coasts are one of the main departure points for thousands of migrants heading to Europe.
The Atlantic route is particularly perilous due to the strong currents, with thousands of deaths and disappearances every year on overloaded, often unseaworthy boats.