A concerted push by Nigeria's military and neighbours Chad, Cameroon and Niger has regained considerable ground in the fight against Boko Haram. The Nigerian army said it has repelled Boko Haram from all but three local government districts in the north-east.

"We have three local governments remaining, Abadam, Kala-Balge and Gwoza, and we are optimistic that with time we will liberate those," Lieutenant-General Tobiah Minimah, the army chief of staff, told reporters.

Waving captured black and white militant flags, soldiers from Niger and Chad celebrated their liberation of the Nigerian town of Damasak from Boko Haram.

The insurgents had seized Damasak with little resistance in November 2014.

Boko Haram has been fighting a six-year insurgency to create an Islamic state and had taken control of large parts of Nigeria's north-east in the past year. It became notorious internationally after kidnapping more than 200 schoolgirls last April.

At the start of this year, Boko Haram controlled around 20 local government areas, a territory the size of Belgium. In recent months, Nigeria has stepped up its campaign against the militants, and, with the help of a regional force that includes Niger and Chad, has retaken dozens of towns.

Tens of thousands of refugees fleeing Boko Haram have crossed into neighbouring Niger. Among the world's poorest countries, Niger has suffered from recurrent poor harvests, and says it will struggle to feed its people as well as the refugees.