French President Emmanuel Macron has described a coup that ousted Niger's elected leader as "dangerous" for the Sahel on Friday, as Western powers scramble to preserve a key ally in the insurgent-stricken region.

Macron, speaking during a visit to Papua New Guinea, said it was imperative that the constitutional order be restored and vowed to support regional groupings like ECOWAS in mediation or sanctions against putschists.

"This coup is completely illegitimate and profoundly dangerous, for Niger and the whole region," Macron said, while also calling for the release of President Mohamed Bazoum.

Bazoum has been confined to his residence since Wednesday by his own presidential guard.

Niger's government had been seen by many in the international community as a bulwark against Islamist militancy in a vast, arid region that is beset by security challenges.

French and UN troops were in recent years forced to withdraw from neighbouring Mali, but Paris still has 1,500 soldiers in Niger.

Bazoum's overthrow could put the future of their deployment in doubt.