0353 GMT July 17, 2018
President Macky Sall condemned what he called an "armed attack of rare barbarity" and summoned his national security council, a government statement said hours after the attack on Saturday, AFP wrote.
The youths were collecting wood in the Bayotte forest, 20 kilometers from the regional capital Ziguinchor, "when they were attacked by an armed band of 15 people", army spokesman Abdou Ndiaye said.
A source in Ziguinchor said that 13 were killed and two were able to escape, with Ndiaye adding that seven others were injured in the attack.
The Senegalese Press Agency said the assailants would have passed a buffer zone between the Senegalese Army and separatist rebels of the Movement for Democratic Forces in Casamance (MFDC).
Sall "ordered that the perpetrators of this criminal act be found and brought to justice," the government statement said, adding that the ministerial delegation would "evaluate the security situation and offer the nation's condolences to the families".
The rebels began fighting for independence in December 1982 but have long ceased their frequent attacks on soldiers, who retain a visible presence in the area.
The army deployed 150 troops to evacuate the victims and flush out the perpetrators, while the dead were taken to a hospital morgue.
Casamance, separated from the rest of Senegal by The Gambia, has been calm for several years since Sall took power in 2012, though peace talks have failed to yield a definitive settlement.