Niamey: General Abdourahmane Tchiani, who often goes by "Omar," appeared on state television as the leader of the soldiers who staged the coup that began on Wednesday.
Elected President Mohamed Bazoum has been detained by army putschists since Wednesday morning.
The United States condemned the military coup in Niger and may cease security cooperation with the country and the EU has threatened to cut aid to Niger.
The US State Department said that Antony Blinken had called Niger's ousted President Mohamed Barzoum for the second time in as many days.
Blinken offered Bazoum Washington's "unflagging support," department spokesman Matt Miller said in a statement, while warning those detaining him that "hundreds of millions of dollars of assistance" was at risk.
The US secretary of state made the call while wrapping up a Pacific tour spanning several countries.
Blinken also "praised Bazoum's role in promoting security not only in Niger but the wider West Africa region," and said Washington would keep working to "ensure the full restoration of constitutional order and democratic rule in Niger."
He also discussed the developing situation in Niger with former Nigerien leader Mahmadou Issoufou, expressing concern over Bazoum's continued detention, and with French Foreign Minister Catherine Colonna.
France and the US are both heavily involved in Niger, supporting its fight against jihadist insurgents. Niamey's importance had increased since coups in neighboring Mali and Burkina Faso, struggling to contain the same militants in the tri-border area, led to the exit of Western forces and the entry of mercenaries from the Russian Wagner Group in recent years.