The French army faces repatriating its equipment mostly overland through Chad and then Cameroon -- a distance of more than 3,000 kilometres (around 1,860 miles) some of which is known to harbour jihadist groups.

"Chad has agreed to offer a corridor of its territory for the return of French troops to France," chief of general staff of the Chadian army Abakar Abdelkerim Daoud said in a statement dated Thursday.

"Chadian forces will escort these convoys from the Nigerien border to N'Djamena for the airport... and to the Cameroonian border for the port of Douala."

Roughly 1,400 soldiers were based in the capital Niamey and western Niger to battle fighters linked to the Islamic State group and Al-Qaeda, bringing with them vast quantities of gear.

A first contingent set off from their forward base in Ouallam for neighbouring Chad on Tuesday, travelling by road in armoured vehicles under Nigerien escort for the journey of over 1,600 km.

From N'Djamena, French troops can leave by air with their most sensitive equipment.

However, most will have to be moved by land and sea, said a former French military logistics specialist in Africa who asked to remain anonymous.

Niger's land borders with Benin and Nigeria have been closed since military leaders ousted democratically elected president Bazoum in a coup on July 26.