Gunmen shot dead 13 people and wounded seven others on Saturday in the forest of Senegal's southern region of Casamance, the army said.
It was not clear who was responsible or what the motive was for the attack on a group of civilians searching for firewood near the town of Borofaye and about 10 km (six miles) from the Guinea Bissau border, said army spokesman Abdoul Ndiaye.
The area is home to separatist rebels who in 1982 formed what is now one of Africa's oldest rebellions in their search for independence from northern Senegal.
The Movement of Democratic Forces for Casamance (MFDC) is active in the area of southern Senegal, which is separated from the more affluent north by Gambia.
The movement has been largely dormant since a ceasefire in 2014. However, many areas remain off-limits because of land mines and arms stockpiles.
President Macky Sall called in his New Year address for dialogue with the rebels and a lasting peace.
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