Nigerian forces backed by airstrikes seized the northeastern border town of Baga from Islamist group Boko Haram on Saturday, the military said, a significant victory in an offensive against an insurgency affecting four African states.

Retaking the town, at Nigeria's border with Chad, Niger and Cameroon, had been a key objective, given that it had served as the headquarters of a multinational force of troops from all four countries.

"We have secured Baga. We are now in full control. There are only mopping up exercises left to do," Defense Spokesman Maj. Gen. Chris Olukolade said by telephone.