– Boko Haram hasn't only killed thousands of people in its campaign to impose Islamic law, or sharia: it's wrecking agriculture in some of the country's main food-growing areas.

With farmers afraid to go to their fields in Nigeria's northeast, rains that exceeded expectations failed to translate into a better harvest.

"No one can move a kilometer due to fear," Abba Gambo, an agricultural science lecturer at the University of Maiduguri, in the Borno state capital, said in an interview. "Most of them have fled their homes."

More than 1.5 million people, mostly farmers, have been forced to flee their homes as Boko Haram intensified its insurgency in the past year, according to the U.N.High Commissioner for Refugees. The worst-hit states of Borno, Yobe and Adamawa produce such staple foods as cowpeas, rice, millet, sorghum, corn and yams, as well as tomatoes, onions, fish and livestock.

Boko Haram, which translates as "western education is a sin" from the local Hausa language, is entering the sixth year of a campaign to impose sharia, or Islamic rule, in Africa's biggest oil producer and most populous country of more than 170 million people.

At least 13,000 people have died in gun and bomb attacks carried out by the group across northern Nigeria and the capital, Abuja, according to the government.

Some of the worst attacks have occurred around Lake Chad, which supports fisheries and has alluvial soils that yield bumper harvests.

The town of Baga, which in January was razed in an attack that Amnesty International said claimed hundreds of lives, was built on fish and crop trade.

Lake Chad borders four countries, including Cameroon and Niger. Out of more than 200 villages on the Nigerian side of the water body, about 140 have been ransacked by the Islamist militants, said farmer Mala Duguri in a phone interview. Most of them grew onions, water melons, wheat, rice and cowpeas.

"The soil is very fertile but unfortunately we are in a state of fear," Duguri said.

Where farmers are still able to produce, they face difficulties moving their harvest to the towns and cities where they're in demand because key bridges on important link routes have been sabotaged, and vehicles traveling on remote roads risk being ambushed.

A bridge linking Nigeria and border communities near Cameroon and another connecting Maiduguri in Borno state to Damaturu in Yobe state were both blown up by Boko Haram last year, disrupting transportation in the region.

"Before this Boko Haram insurgency, we used to load 40 trucks with fish to the south every week on market days," Mohammed Sani, a fish seller in Maiduguri, said. "But now it has reduced to five trucks; maximum seven a week."

There is an urgent need for the government to reassert its authority in the areas now threatened by the Islamist insurgency to make them safe for farmers to return to the fields in order to secure food supplies, said Gambo of the University of Maiduguri

"There is looming food insecurity, and definitely prices will go up because ­supply will fall short," Gambo said.