Chad and Niger launched a joint army operation against Boko Haram militants in Nigeria on Sunday, CNBC quoted military sources as saying.
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 Daesh.
Chad, backed by its air force, has already sent troops many kilometres inside northeastern Nigeria, winning back areas from the Sunni jihadist group near the Cameroon border.
"We can confirm that Chadian and Nigerien forces launched an offensive this morning from Niger. The offensive is underway," said Colonel Azem Bermandoa, spokesman for Chad's army.
One of the Niger sources said Abuja had given the green light for the operation.