Gunmen have killed at least 13 people in an attack in Senegal’s volatile south, military sources say. Security officials in the West African country said the massacre took place outside the town of Ziguinchor in the Casamance region late Saturday.

The victims were reportedly collecting firewood in the region “when they were attacked by an armed band of 15 people,” with hospital sources saying that some of the victims had been shot and others decapitated.

Army spokesman Colonel Abdoul Ndiaye confirmed the number of fatalities and said seven others had been wounded in the deadly attack, adding that the military had stepped up its presence in the area.

The Casamance region in southern Senegal has been ravaged by a long-running armed conflict between government forces and separatist groups.

While there was no immediate claim of responsibility for the killings, suspicion has fell on members of a separatist group known as the Movement of Democratic Forces of Casamance (MFDC), which has been seeking independence in Senegal’s south for more than three decades.

The deadly assault came hours after the release of two prisoners belonging to the rebel outfit. It sparked fears of renewed unrest in the area as Casamance had been relatively calm since 2012 despite the presence of separatists.

The armed wing of the MFDC group agreed to a ceasefire in 2014, and the last major attack blamed on the group was in 2013, when rebels took hostage a dozen employees of a South African bomb disposal firm. The victims, all Senegalese citizens, were eventually released.