Chad and Niger launched a joint army operation against Boko Haram militants in Nigeria on Sunday, military sources said, stepping up a regional push to end the Sunni Muslim group's six-year insurgency.
Boko Haram has expanded cross-border raids into Cameroon, Chad and Niger in recent months. This has spurred Nigeria's neighbours to retaliate, although cooperation between them and Abuja has been limited and at times strained.
Sunday's strike marks Niger's first major push into Nigerian territory to combat Boko Haram. It had until now only defended itself against militant incursions in border areas.
Nigeria's government appealed for greater international help in combating the insurgency after Boko Haram pledged allegiance on Saturday to Islamic State, the violent jihadi organisation that rules a self-declared caliphate in parts of Iraq and Syria.