Chadian troops have entered the northern Nigerian town of Gambaru on the border with Cameroon that has been under the control of Nigeria's Boko Haram Islamist insurgents for several months, Chadian military sources said on Tuesday.

The Nigerian government and military officials were not immediately available to comment.

"Our troops entered Nigeria this morning. The combat is ongoing," one of the sources at Chad's army headquarters told Reuters.

Chad has deployed some 2,500 troops as part of a regional effort to take on the militant group that has waged a bloody insurgency to create an Islamist emirate in northern Nigeria.