Niger, Chad and Cameroon are seeking to pin down Boko Haram within Nigeria's borders ahead of a ground-and-air offensive by a regional taskforce, due to start from the end of next month, a senior Niger military official told Reuters.

The Islamist group, which has killed thousands of people in a six-year insurgency in Nigeria, has fought fierce battles with the three countries' armies in southern Niger and northern Cameroon, near Nigeria's borders, in recent weeks.