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.

The operation, which follow a sustained buildup of troops in southern Niger, opens up a new front in regional efforts to wipe out the Islamist group, whose six-year insurgency has spread across borders. It also follows Boko Haram's pledge of allegiance to the Islamic State group at the weekend.

Sunday's operation marks Niger's first major push into Nigerian territory to combat Boko Haram.