The Economic Community of West African States(ECOWAS) leaders have threatened military action against Niger’s junta after it took power in a coup last week.
The leaders gave the junta seven days to reinstate President Mohamed Bazoum, who is being held captive.
Earlier, the junta warned it would resist any “plan of aggression against Niger” by regional or Western powers.
Meanwhile, hundreds of coup supporters protested outside the French embassy in the capital Niamey.
Leaders from Ecowas, the bloc of West African nations, held crisis talks in Nigeria’s capital Abuja on Sunday to discuss the latest coup – which follows army takeovers in neighboring Mali and Burkina Faso.
A statement read out after the summit said that Ecowas had “zero tolerance” for coups.
The regional bloc would “take all measures necessary to restore constitutional order” if its demands were not met within a week.
“Such measures may include the use of force,” and military chiefs are to meet “immediately” to plan for an intervention, the statement added.
The Special Representative and Head of the United Nations Office for West Africa and the Sahel was at the meeting and said Ecowas had taken decisive action because events in Niger were concerning.
“Niger is playing a key role in fighting terrorism. If Niger stop playing this role this will give more space and more leeway to terrorists to expand in the region,” Dr Leonardo Santos Simao told BBC’s Newshour program.
He added that “no official negotiations” were taking place between Ecowas and the country’s military junta.
This is the first time Ecowas has threatened military action to reverse the coups that have taken place in the region in recent years.
It last sanctioned military intervention in 2017, when Senegalese troops were deployed to The Gambia to force long-serving ruler Yahya Jammeh to leave office after he refused to accept defeat in elections.
Chad’s President Mahamat Idriss Déby Itno has gone to Niamey to tell the junta to step down, Chad’s government said.
He is the first foreign leader to visit Niger since the coup and has met junta deputy leader Gen Salifou Mody.
It is unclear whether he will hold talks with Gen Abdourahmane Tchiani, the head of the presidential guards unit who has declared himself Niger’s new ruler.
The West African leaders also announced the immediate enforcement of a no-fly zone over Niger for all commercial flights, the closure of all land borders with the country, and the imposition of financial sanctions against the junta.
Ahead of their meeting, Gen Tchiani warned Ecowas and unnamed Western nations against stepping in.
“We once again reiterate to Ecowas or any other adventurer our firm determination to defend our fatherland,” the statement, which was read out on TV, said.
The coup has prompted concern that Niger, a former French colony, could pivot towards Russia.
The ousted president had worked closely with both regional and Western nations to fight militant Islamists.
Burkina Faso and Mali moved closer to Russia after their own coups.
In Niamey, some of the protesters outside the French embassy chanted “Long live Russia”, “Long live Putin” and “Down with France”, AFP news agency reports.
They also set fire to the walls of the embassy compound.
France would not tolerate any attack on its interests in Niger, and would respond in an “immediate and intractable manner”, President Emmanuel Macron’s office said in a statement.
Niger’s coup has been condemned by Western nations, but welcomed by the leader of Russia’s Wagner mercenary group Yevgeny Prigozhin, who 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,” he was quoted as saying on a Wagner-affiliated Telegram channel, although his comments have not been independently verified.
In Mali, the junta has brought in Wagner to help it fight militant Islamists.
France announced the withdrawal of its troops last year amid growing hostility from the junta.
It subsequently moved its regional military headquarters to Niger.
In June, Mali’s junta said the UN’s 12,000 peacekeepers also had to leave following a decade of countering Islamist militants.
The UN agreed, saying the withdrawal would be completed by the end of the year.
On Saturday, France said it had suspended all development aid and budgetary support to Niger. The European Union and the US have made a similar decision.