DAKAR, Senegal (AP) — At least 40 farmers were killed in an attack by Islamic militants in northeast Nigeria’s Borno state, a government official said Monday.

The attack Sunday was suspected to have been carried out by extremists from the Boko Haram group and its breakaway faction that is loyal to the Islamic State group in Borno’s Dumba community, said Borno state Gov. Babagana Umara Zulum.

He warned civilians to stay within designated “safe zones” that have been cleared by the army of both extremists and munitions.

Zulum also called for an investigation into the attack by the armed forces.

"Let me assure the citizens of Borno that this matter will be thoroughly investigated for further necessary action. Let me use this opportunity to call on the armed forces to track and deal decisively with the perpetrators of this heinous act of violence against our innocent citizens,” he said.

Boko Haram, Nigeria’s homegrown jihadis, took up arms in 2009 to fight Western education and impose their radical version of Islamic law. The conflict, now Africa’s longest struggle with militancy, has spilled into Nigeria’s northern neighbors.

Some 35,000 civilians have been killed and more than 2 million have been displaced in the northeastern region, according to the U.N. The 2014 kidnapping of 276 schoolgirls by Boko Haram in the village of Chibok in Borno state — the epicenter of the conflict — captured the attention of the world.