Nigeria, Africa's most populous nation and largest oil producer, is roughly divided between a mainly Muslim north and predominantly Christian south.

The limitations of the authorities were recently highlighted when the alleged mastermind of a Christmas Day attack outside a church that killed 44 people escaped police custody in suspicious circumstances.

Attacks specifically targeting Christians have also given rise to fears of a wider religious conflict in the country, with Christian leaders warning they would defend themselves. Some have even evoked the possibility of civil war.

However, attacks blamed on Boko Haram have included a wide range of targets, including Muslims.

The group also claimed responsibility for the August suicide bombing of United Nations headquarters in the capital Abuja that killed 25 people.