The Senegalese government vowed Sunday to be "relentless" in tracking down and bringing to justice those responsible for the execution of 13 people in the southern region of Casamance.

Gunmen ordered a group of men and youths, out looking for wood, to lie on the ground deep in the forest before opening fire, a survivor said following the first upsurge in violence in the isolated Senegalese region in years.

President Macky Sall, condemning an "armed attack of rare barbarity", summoned his national security council and ordered a ministerial delegation to the scene.

"A hard and relentless hunt will be conducted to find the perpetrators of this despicable act," interior minister Aly Ngouille Ndiaye told Senegalese press agency APS during a visit to survivors at the hospital in Ziguinchor, the region's capital.

The attack happened on Saturday in Borofaye forest in the commune of Boutoupa-Camaracounda.

According to the government, ten of the 13 killed were shot dead, two were stabbed to death and one was burned. Half a dozen more were wounded, with the most seriously hurt being transferred to Dakar for treatment.

- Shot in cold blood -

The as yet unidentified group stopped the young men and rounded them up before shooting them coldly, survivors told AFP.

"They made us lie face down and started firing," a 45-year-old father of two with two wives said Sunday at Ziguinchor Ayib Ly hospital as he received treatment for injuries to his back and foot.

"Injured people trying to escape were finished off" by the attackers, said another survivor, Amadou Diallo.

Ibrahima Daffe, described being hit by "two bullets in the back", fired by men "in military uniforms", wearing "rangers" type boots and speaking the local language.

A source in Ziguinchor said 13 youths were killed, while army spokesman Abdou Ndiaye told AFP seven others were injured.

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).

Abdoulaye Balde, a former armed forces minister and deputy mayor of Ziguinchor, told AFP those responsible were likely "not fully associated with the peace process" in a region where rebels began fighting in December 1982 -- though they have long ceased attacks on soldiers.

"When you negotiate with the main (MFDC) leaders, dissidents are often not involved," said Balde, who called the attack a surprise while noting that "we knew peace was precarious" in the tense region.

Sall has "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 army deployed 150 troops to evacuate the victims and flush out the attackers, and a resident of the nearby village of Bourafaye Bainuk said shots were heard as the army undertook a "deep search".

- Elusive peace deal -

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.

Ziguinchor governor Guedj Diouf said he thought the attack would not derail the peace process. "There's an irreversible dynamic in the direction of peace," Diouf said in remarks carried by the news website PressAfrik.

Former colonial power France, whose tourists visit other areas of Senegal in droves, removed Casamance from its list of danger zones in October 2016.

The attack came a day after the army released two MFDC fighters following negotiations spearheaded by Rome's Community of Sant'Egidio, a charity with ties to the Vatican specialising in peace mediation.

The Casamance separatist movement left thousands of civilians and military personnel dead and forced many to flee over three decades, as well as hurting the economy dependent on agriculture and tourism.

In a New Year's message last Sunday, Sall appealed to the Casamance rebels to continue talks to create a "definite peace".