The head of Niger's presidential guard, General Abdourahamane Tiani, has appeared on state television as the president of the transitional council that seized power in a coup.
The guard instigated the coup on Wednesday and detained President Mohamed Bazoum in the presidential palace.
A group of soldiers who later appeared on state television said they had stripped Bazoum of power.
Colonel Amadou Abdramane, who announced the coup, said the military had acted in response to deteriorating security and bad governance.
Abdourahamane Tiani reiterated that soldiers seized power due to the worsening security.
He also criticised the non-co-operation with military governments in Burkina Faso and Mali in the fight against insurgencies in the region.
Insecurity has remained a problem since Bazoum was elected in 2021 as jihadists that took root in Mali in 2012 gained ground, killing thousands and displacing more than six million across the Sahel.
Niger is a key ally of Western countries against Islamist insurgencies in West Africa and a number of foreign troops are based there, including French and American.
French President Emmanuel Macron said on Friday he was prepared to back sanctions against the perpetrators of the "dangerous" coup after his foreign minister said the power grab did not appear to be definitive.
Former colonial power France has made Niger the cornerstone of its more than decade-long counter-insurgency operations against Islamist militants in the Sahel region.
It has about 1500 soldiers in the country who support the local military, having redefined its strategy after thousands withdrew from neighbouring Mali and Burkina Faso following coups there.
If Wednesday's coup in Niamey succeeds, French troops could well be forced to withdraw from there too, diplomats and analysts said.
"This coup d'etat is completely illegitimate, extremely dangerous for Nigeriens, for Niger and the entire region," Macron told a news conference in Papua New Guinea.
Macron said he would support West African regional bloc ECOWAS should it decide to impose sanctions on those behind it.
"If you hear me say attempted coup d'etat, it's because we don't consider that things are definitive," French Foreign Minister Catherine Colonna told reporters in PNG.
Macron said he had spoken to Bazoum, who is being held in his palace, and called for him to be reinstated.
France has a further 1000 troops based in Chad, where it has been less critical of a delayed transition to civilian rule after a 2021 coup.
France has faced a growing wave of resentment towards its influence in the Sahel, anger that some anti-Western elements have sought to stoke.
Yevgeny Prigozhin, boss of Russian mercenary group Wagner, on Friday hailed the coup as a liberation from Niger's Western colonisers, although the foreign minister in Moscow, Sergei Lavrov, said on Thursday constitutional order should be restored.
Since Wednesday, a spate of anti-French rhetoric and misinformation regionally and linked to Russia has sought to stoke anger against Paris over its activities in Niger, including accusing it of pillaging resources to fuel its nuclear reactors.
French nuclear company Orano operates uranium mining sites in Niger's north, an area prone to security threats.
It said on Friday its operations were continuing as normal and that French nuclear power plants sourced less than 10 per cent of their uranium from Niger.
