The EU and France have cut off financial support to Niger and the US has threatened to do the same after military leaders this week announced they had overthrown the democratically elected president Mohamed Bazoum.
Niger is one of the poorest countries in the world, receiving close to $2 billion a year in official development assistance, according to the World Bank.
It is also a security partner of former colonial power France and the US, which both use it as a base to fight an Islamist insurgency in West and Central Africa's wider Sahel region.
Niger's foreign allies have refused to recognise the new military government led by Gen Abdourahamane Tiani, previously head of the presidential guard, who officers declared head of state on Friday.
Mr Bazoum has not been heard from since early on Thursday when he was confined within the presidential palace, although the EU, France and others say they still recognise him as the legitimate president.
“In addition to the immediate cessation of budget support, all co-operation actions in the domain of security are suspended indefinitely with immediate effect,” EU foreign policy chief Josep Borrell said.
The French Foreign Ministry said France had suspended all development aid and budget support with immediate effect, demanding a prompt return to constitutional order with Mr Bazoum back in charge.
French development aid for Niger was around €120 million ($130 million) in 2022, and was expected to be slightly higher this year.
US Secretary of State Antony Blinken said: “The very significant assistance that we have in place for people in Niger is clearly in jeopardy.”
The US has two military bases in Niger with about 1,100 soldiers and also provides hundreds of millions of dollars to the country in security and development aid.
Niger is a key partner of the EU in helping curb the flow of irregular migrants from sub-Saharan Africa. The EU also has a small number of troops in Niger for a military training mission.
The bloc allocated €503 million from its budget to improve governance, education and sustainable growth in Niger over 2021-2024, according to its website.
Through its European Peace Facility, the EU had approved around €70 million to support Niger's armed forces since July last year, including €4.7 million of weapons approved on June 8.
The UN said the coup has not affected its deliveries of humanitarian aid.
It was difficult to assess how much support the coup had among the population. Some crowds came out in support of Mr Bazoum on Wednesday, but the following day coup supporters also took to the streets.
The Economic Community of West African States and the West African Monetary Union will hold emergency summits in Nigeria on Sunday to discuss the situation, and further economic and financial sanctions could be decided.
After an emergency meeting on Friday, the African Union demanded the military return to their barracks and restore constitutional order within 15 days. It did not say what would happen after that.