Chadian soldiers killed 13 fighters from Islamist militant group Boko Haram in a battle in the Nigerian town of Gambaru on Wednesday, the army said in a statement, adding that one Chadian soldier was killed.

Chad's army is in Gambaru as part of a regional offensive including Niger and Cameroon against Boko Haram, whose militants have staged cross-border raids in addition to attacks inside Nigeria as part of a campaign to establish a caliphate.

Much of the recent fighting has taken place near Lake Chad, which borders Nigeria, Chad, Cameroon and Niger, and in northern Cameroon itself.

Hundreds of people have been killed in the incursions as well as in Nigeria, according to military sources from the different countries. There is no independent verification of the casualty toll.

"We knew they were going to attack us. We were waiting. The battle didn't last long. They fled," one military source told Reuters of the Gambaru fighting.

The army said 11 Chadian soldiers were wounded, three out of 14 Boko Haram vehicles were destroyed, and one was seized.

Boko Haram attacked the town of N'Guigmi on Niger's border with Nigeria late on Tuesday with mortars and machine guns, but the attack failed, a Niger military source said.

"The situation is precarious because Boko Haram could come back to assault us at any moment," a military source said on Wednesday, adding that schools in the town are closed.

N'Guigmi lies near Lake Chad, around 100 km (60 miles) east of Diffa, where Niger declared a 15-day state of emergency and curfew on Tuesday.

Militant fighters were harassing Niger's army with mortars, but the army was able to locate the source of the firing and neutralise the threat, said a military source.

Authorities in Diffa searched house-to-house for Boko Haram suspects on Wednesday and arrested four men who arrived on a truck bound for Niger's second largest city, Zinder, a military source said.

Schools in the town were closed and civil servants had left their posts to take their families to more secure towns elsewhere, said a police officer in the town, the source said.

Niger's parliament this week unanimously approved sending troops to northern Nigeria as part of the regional offensive.