The United States has offered its “unflagging support” to Niger’s ousted president Mohamed Bazoum.
Mr Bazoum was removed in a coup this week led by Gen Abdourahmane Tchiani, also known as Omar Tchiani, head of the presidential guards unit.
US Secretary of State Antony Blinken warned those detaining Mr Bazoum that “hundreds of millions of dollars of assistance” was at risk.
Gen Tchiani had earlier declared himself the country’s new leader.
Mr Bazoum is Niger’s first elected leader to succeed another since independence in 1960.
He had been considered a key ally by Western nations in the fight against Islamist militants in the region. He is currently thought to be in good health, and still held captive by his own guards.
Mr Blinken called Mr Bazoum for the second time in as many days, saying that Washington would keep working to “ensure the full restoration of constitutional order and democratic rule in Niger”.
In a separate call to Mahamadou Issoufou – who was Niger’s president before Mr Bazoum – Blinken said he “regretted that those detaining Bazoum were threatening years of successful cooperation and hundreds of millions of dollars” in assistance, department spokesman Matt Miller said.
The UN Security Council called for the immediate and unconditional release of Mr Bazoum and condemned the “efforts to unconstitutionally change the legitimate government” of Niger.
France, whose colonial empire included Niger, has said that it does not recognise any of the coup’s leaders and will only recognise Mr Bazoum as head of state.
“We reiterate in the strongest terms the international community’s clear demand for the immediate restoration of constitutional order and democratically elected civilian power,” a statement from the French foreign ministry read.
The coup has plunged the Sahel region into further uncertainty after similar takeovers in Burkina Faso and Mali.
It has been roundly condemned by international bodies including the African Union, West African regional bloc (Ecowas), the EU and the UN.
However, the leader of Russia’s Wagner mercenary group has reportedly praised the coup, describing it as a triumph.
“What happened in Niger is nothing other than the struggle of the people of Niger with their colonisers,” Yevgeny Prigozhin was quoted as saying on a Wagner-affiliated Telegram channel.
The BBC has not been able to verify the authenticity of his reported comments.
The Wagner group is believed to have thousands of fighters in countries including the Central African Republic (CAR) and Mali, where it has lucrative business interests but also bolsters Russia’s diplomatic and economic relations. Wagner fighters have been accused of widespread human rights abuses in several African countries.