Niger military officers deposed President Mohamed Bazoum on July 26 and have defied calls from the United Nations, ECOWAS and Western powers to reinstate him, prompting West African heads of state to order the standby force to be assembled.

"Let no one be in doubt that if everything else fails, the valiant forces of West Africa...are ready to answer to the call of duty," ECOWAS Commissioner for Political Affairs, Peace and Security Abdel-Fatau Musah said.

"By all means available, constitutional order will be restored in the country," he told assembled defence chiefs from member countries, listing past ECOWAS deployments in Gambia, Liberia and elsewhere as examples of readiness.

Niger has strategic importance beyond West Africa because of its role as a hub for foreign troops involved in the fight against Islamist insurgents in the Sahel region and its uranium and oil reserves.

Western countries fear the junta could follow the lead of neighbouring Mali, where the military government threw out French troops and instead invited in mercenaries from Russia's Wagner group, which has welcomed the coup in Niger.