Chad and Niger have launched a joint army operation against Boko Haram fighters in Nigeria, military sources say.

A source from the Niger government said the two countries had launched a "ground and air" operation against Boko Haram in northeastern Nigeria.

"We can confirm that Chadian and Nigerian forces launched an offensive this morning from Niger. The offensive is under way," Colonel Azem Bermandoa, spokesperson for Chad's army, said on Sunday.

The offensive marks Niger's first major push into Nigerian territory to combat Boko Haram. It had until now only defended itself against incursions in border areas.

Chad, backed by its air force, has already sent troops many kilometres inside northeastern Nigeria, winning back areas from the group near the Cameroon border.

So far more than 30 towns and villages have been retaken from Boko Haram since Chad and Niger joined efforts by Nigerian and Cameroonian troops to fight the group.

The fresh 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).

Trade damaged

"For Chad, Niger and Cameroon, defeating Boko Haram is crucial. Five years of violence has cut of much of trade between them and Africa's biggest economy, Nigeria," Al Jazeera's Ahmed Idris, reporting from Abuja, said.

"There is also the fear that the group, if left unchecked, will expand its area of control and become a bigger threat for the entire region."

The regional 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 at markets and a bus station in the northeastern Nigerian town of Maiduguri.

Aliyu Musa, a UK-based specialist on Boko Haram, said the group had found itself stuck between fire from Nigeria and multinational forces and were now looking for "soft targets".

"That is why they are attacking bus stations where they find civilians that are kind of unaware," he told Al Jazeera.

"It will continue like this until finally the forces are able to force them to succumb."

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.