At least 40 farmers were killed over the weekend in an attack by suspected Islamist militants, the governor of Nigeria's northeastern Borno state said on Monday. 

What authorities said about the attack

Governor Babanga Umara Zulum said Sunday's attack was thought to have been carried out either by Boko Haram extremists, or members of its breakaway faction that's loyal to the so-called Islamic State West Africa Province (ISWAP). 

He called for an investigation and retaliation.

"Let me assure the citizens of Borno that this matter will be thoroughly investigated for further necessary action," Zulum said. "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 warned civilians to stick within designated "safe zones" that have been cleared of militants and munitions by the military. 

The state's commissioner of information, Usman Tar, said the farmers had strayed outside one such security corridor, venturing into an area known for insurgent activity and minefields.

Attacks on famers common, exacerbating food shortages

Much of the Muslim-majority northeastern state of Borno, the heartland of the Boko Haram movement that took up arms in 2009, is not fully controlled by Nigerian authorities. 

In recent months, attacks, kidnappings and theft from rural farmers by Islamist militants have been commonplace. 

As authorities urge farmers to stick to designated safe zones and deploy rangers to try to protect them, some lament having to leave more fertile soil in more perilous areas untended. 

The attacks on farmers have exacerbated food shortages in the impoverished state, which was also hit by flooding last September after a dam collapsed, temporarily displacing more than half of the population of the state capital Maiduguri.

The violence has also spilled over into neighboring countries like Chad, which borders Borno state.

msh/lo (AFP, AP)