Boko Haram insurgency against African nations
Chad and Niger forces descend on Boko Haram after militant group pledges allegiance to Isis Reuters file photo

Chad and Niger have launched a coordinated ground and aerial offensive against Boko Haram militants who have pledged allegiance to the Islamic State (Isis).

African officials have declined to directly attribute the intensified anti-terror operation to Boko Haram's announcement of loyalty to the IS. But the offensive comes shortly after an audio recording of Boko Haram leader Abubakar Shekau was released online hailing IS chief Abu Bakr al-Baghdadi as the group's "caliph".

"We can confirm that Chadian and Nigerien forces launched an offensive this morning from Niger. The offensive is under way," said Colonel Azem Bermandoa, a spokesperson for the Chadian army.

The campaign is focussed on key areas in the Niger-Nigeria border region. The latest offensive follows the build-up of African troops in the area over several weeks, especially in southern Niger.

About 200 vehicles including machine gun-mounted vehicles were seen moving towards the border region along with tanks.

"Early this morning, troops from Niger and Chad launched ground and air raids against Boko Haram into Nigeria, and the operation is still continuing. It is an intensive operation that is aimed at pulverizing Boko Haram and crippling their capability," a military official in the Niger border town of Diffa told CNN.

Local reports suggest the air strikes have pushed back Boko Haram fighters to their strongholds in Nigeria.

Nigeria had been calling for greater assistance from regional powers and international forces to help defeat Boko Haram.

In recent weeks, the group has stepped up its attacks, posing a massive security challenge to poorly-equipped Nigerian forces.