Soldiers in the West African country of Niger have announced a coup on national TV.
They said they had dissolved the constitution, suspended all institutions and closed the nation’s borders.
Niger President Mohamed Bazoum has been held by troops from the presidential guard since early on Wednesday.
He was promised Washington’s “unwavering support” in a call from US Secretary of State Antony Blinken.
UN Secretary General Antonio Guterres also said he had spoken to the president and offered the UN’s full support.
Mr Bazoum is a key Western ally in the fight against Islamist militancy in West Africa.
Two neighbouring countries, Mali and Burkina Faso, have experienced coups triggered by jihadist uprisings in recent years.
In both countries the new military leaders have fallen out with France, the former colonial power, which also formerly ruled Niger.
Mr Bazoum’s whereabouts are unclear but in a statement on social media platform Twitter on Thursday morning, he said the “hard-won gains will be safeguarded” and that Nigeriens who love democracy will see to it.