The military, backed by regional African forces, have in recent months sought to regain territory lost to the fighters [AP]

"Ground and air" operation in northeastern Nigeria begins a day after the armed group pledged allegiance to ISIL.

The armies of Niger and Chad have launched a "ground and air" offensive against the Boko Haram armed group in northeastern Nigeria, a source from the Niger government said.

"An offensive is under way against Boko Haram," the source told AFP news agency. "Very early [on Sunday] morning, the troops from Niger and Chad began an offensive against Boko Haram... in the area of Bosso and near to Diram."

A resident of Diffa, located in Niger near the Nigerian border, told AFP he saw troops headed toward the frontier early Sunday followed by the sounds of heavy arms fire.

"After some time, the detonations grew further away, an apparent sign that the troops were moving inside Nigeria," he said.

Privately owned radio station Anfani, based in Diffa, reported more than 200 vehicles, including those equipped with machine guns as well as tanks, ambulances, water tankers and transport trucks, in a convoy moving toward the Nigerian border.

It also reported that aircraft had targeted Boko Haram positions on Saturday and early Sunday.

An aid worker told AFP heavy arms fire was coming from the direction of the Doutchi bridge connecting Niger to Nigeria on Sunday morning.

The offensive comes after the African Union on Friday endorsed the creation of a regional force of up to 10,000 men to join the fight against the group which on Saturday pledged allegiance to the Islamic State of Iraq and the Levant (ISIL).

The force, the idea for which was adopted at an AU summit in January, will be based in Chad's capital N'Djamena, the pan-African bloc's Peace and Security Council said.

It will be mandated "to prevent the spread of Boko Haram activities and other terrorist groups" and "eradicate their presence", the body agreed in a meeting earlier week.

Diplomats said Chad, Nigeria, Cameroon, Niger and Benin had committed to providing troops, who would "operate freely" in a still-undefined region.

On Saturday, at least 51 people were killed and more than 100 injured in three bombings in the northeastern Nigerian town of Maiduguri.

Saturday's triple blasts were the worst attack on the city since suspected Boko Haram fighters failed in their bid to seize it at the end of January.