The leaders of a 15-nation West African bloc have given Niger's junta a week to return power to the country's civilian authorities or face consequences, including the use of force.
The regional grouping called the Economic Community of West African States (ECOWAS), issued the threat at an emergency meeting in Nigeria on Sunday.
Niger's military has been holding the country's President Mohamed Bazoum since last week in the seventh coup to hit Africa's Sahel region in recent years. General Abdourahamane Tiani, the head of the powerful presidential guard, has declared himself leader.
In its statement, ECOWAS said unless the military hands over the power back to Bazoum within a week, the bloc would take "all measures" to restore constitutional order.
"Such measures may include the use of force for this effect," it added, noting that the body's defense chiefs will meet to discuss a potential response to the coup.
The bloc has also slapped financial sanctions on the junta leaders and the country, freezing "all commercial and financial transactions" between member states and Niger, already one of the world's poorest nations.
ECOWAS and the eight-member West African Economic and Monetary Union have also said that, with immediate effect, borders with Niger would be closed, commercial flights banned, financial transactions halted, national assets frozen, and aid ended.
Ouhoumoudou Mahamadou, who was Niger's prime minister under Bazoum's government, said ECOWAS sanctions would be disastrous both economically and socially because the country relies heavily on international partners to cover its budgetary needs.
"I know the fragility of Niger, I know the economic and financial context of Niger having been the finance minister and now prime minister," Mahamadou, who was abroad when the coup occurred, told France 24 television from Paris.
He added, "This is a country that will not be able to resist these kinds of sanctions. It will be catastrophic."
Military officials involved in the coup would be also banned from traveling and have their assets frozen, ECOWAS added.
Niger's former president, Mahamadou Issoufou, whom Bazoum succeeded as the head of state, said he intended to negotiate with the junta to restore Bazoum to the presidency.
"I have undertaken, by various ways, to find a negotiated solution that will allow to free President Mohamed Bazoum and to reinstate him," Issoufou wrote on Twitter.
The junta has condemned the ECOWAS summit, saying its aim was to "approve a plan of aggression against Niger, in the form of an imminent military intervention in [the capital] Niamey."
The intervention would be "in cooperation with African countries who are not members of the regional body and certain Western nations," junta member Amadou Abdramane said on the national television.