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

"An offensive is underway against Boko Haram," the source told AFP. "Very early this morning, the troops from Niger and Chad began an offensive against Boko Haram... in the area of Bosso and near to Diram."

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 group.

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.

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