Unidentified assailants killed 40 people and wounded 33 others in an attack on the army and volunteer defense forces in northern Burkina Faso, the government said in a statement on Sunday.

The attack took place on Saturday in the village of Aorema near the town of Ouahigouya in the North Region, not far from the border with Mali, an area overrun by Islamist groups linked to al-Qaida and Islamic State that have carried out repeated attacks for years.

It is not clear which group carried out the attack. It comes nine days after gunmen killed 44 people in the villages of Kourakou and Tondobi in the north of the West African country.

Six soldiers and 34 members of a volunteer defense force were killed in Saturday’s attack, the statement said. The government has encouraged civilians to join local defense forces to try and stem eight years of violence in which thousands of people have died and millions have been forced to flee their homes.

The unrest in Burkina Faso triggered two coups last year by the military, which has vowed to retake control of the country but has failed to stop attacks.

Unrest in the region began in Mali in 2012, when Islamists hijacked a Tuareg separatist uprising. The violence has since spread into neighboring Burkina Faso and Niger and threatens to destabilize coastal countries further afield.