The US has offered "unflagging support" to Niger's ousted president Mohamed Bazoum after he was ousted in a coup.

US Secretary of State Antony Blinken warned those detaining Mr Bazoum that "hundreds of millions of dollars of assistance" was at risk.

The head of the presidential guards unit Gen Abdourahmane Tchiani has declared himself Niger's new leader.

Mr Bazoum had been considered a key Western ally in the fight against Islamist militants in the region.

There are now concerns in the West about which countries the new leader will align with. Niger's neighbours, Burkina Faso and Mali, have both pivoted towards Russia since their own coups.

Mohamed Bazoum - Niger's first elected leader to succeed another since independence in 1960 - is currently thought to be in good health, and still held captive by his own guards.

Mr Blinken has called him twice and told him Washington would work to restore democratic rule in Niger, a state department spokesman said.

He has also called Mahamadou Issoufou, Niger's president before Mr Bazoum, to tell him the coup threatened "years of successful cooperation" as well as financial aid.

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.

However, the leader of Russia's Wagner mercenary group has reportedly described 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.

"With colonisers who are trying to foist their rules of life on them and their conditions and keep them in the state that Africa was in hundreds of years ago."

He added: "Today this is effectively gaining their independence."

The BBC has not been able to verify the authenticity of his reported comments.

Wagner 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.