ABUJA, Nigeria (AP) — At least 40 civilians were killed last weekend by al-Qaida-linked rebels trying to take control of a besieged town in Burkina Faso’s hard-hit northern region, the United Nations’ rights office said, calling the attack a war crime.

In one of the largest clashes in recent years in the West African nation under threat from fighters linked to al-Qaida and the Islamic State group, a large number of fighters tried to take control of Djibo near Mali’s border.

The town, located 210 kilometres (130 miles) from the capital, Ouagadougou, has been under blockade by rebels for more than a year, often struggling to provide essential services.

The militants in the latest attack, which happened on Sunday, also wounded 42 people and set fire to three camps for internally displaced people, U.N. Human Rights Office spokesperson Seif Magango said in a statement on Tuesday that blamed the attack on JNIM, an umbrella coalition of armed groups aligned with al-Qaida.

“Deliberately targeting civilians or individuals not taking direct part in hostilities constitutes a war crime,” the U.N. department said, citing reports from its workers on the ground.

The Associated Press couldn’t reach witnesses or survivors in the area, which has frequent internet cuts and where the military government is known to crack down on civil society.

State-run RTB Television ran images — which The Associated Press couldn’t verify — that showed large groups of people riding motorcycles as they appeared to flee aerial bombardment.

“Attacks on civilians are inexcusable and must stop, and those responsible must be held to account following thorough, impartial and independent investigations by the authorities,” the U.N. statement added.

Around half of Burkina Faso’s territory remains outside of government control. The landlocked country has been ravaged by jihadi attacks. Fighters have killed thousands and displaced more than 2 million people, further threatening the stability of the country that had two coups last year.