DAKAR, Jan. 6 (Xinhua) -- Thirteen youths were killed and at least six others were injured on Saturday by a group of armed separatist rebels in the southern region of Casamance in Senegal, security authorities said.

The victims were collecting firewood in a forest near the principle town of Ziguinchor in the region when they were attacked, said Abdou Ndiaye, an Senegalese army spokesperson.

The attack was carried out by a group of 15 rebels from the separatist Movement of Democratic Forces of Casamance (MFDC) in the region bordering Guinea Bissau to the south.

The military is devoting full efforts to search for the attackers, Ndiaye said.

On Friday, government forces had released two prisoners belonging to the separatist group following negotiations as the rebel group has agreed to a ceasefire with the government.

The restive area of Casamance has seen armed conflicts between government forces and the MFDC since 1982 as the MFDC rebels have been seeking independence from the West African nation.

The two parties arrived at a peace agreement by the end of 2004, but some hard-line rebels continued to fight with the government. They restarted peace talks in 2013.

Long-time confrontations in the region have claimed thousands of lives of both civilians and soldiers.