The attack took place in a forest close to the village of Diagnon in the Casanace region, about 30km from Ziguinchor, a military source speaking on condition of anonymity told AFP.
The victims were all civilians.
Villagers said about 20 people had gone to work in the forest and several were missing, the official said.
The official blamed the deaths on the separatist Casamance Movement of Democratic Forces (MFDC,) which has been fighting for independence from Senegal since 1982.
The region has seen periods of calm interspersed with heightened tensions. Between the end of 2010 and the beginning of 2011, about 20 troops were killed, according the Senegalese military.
Many suspected rebels are also thought to have died, though there is no official tally of the number.
There is no immediate end in sight for Casamance’s conflict, despite several peace accords between Dakar and the region.