French Foreign Minister Catherine Colonna said there were "ways out" for coup plotters who chose to heed global condemnation of President Mohamed Bazoum's ouster.
The latest target of a coup in Africa's turbulent Sahel region, Bazoum has been confined to his residence since Wednesday by his own presidential guard.
He stood his ground as denunciations of the putsch swelled from African and international organisations and allies Germany and the United States, as well as France.
"The hard-won (democratic) gains will be safeguarded," Bazoum said on Twitter, which is being rebranded as X.
Colonna said Friday that Bazoum was in "good health" and had spoken to President Emmanuel Macron.
"If you hear me talking about an attempted coup, it's because we don't consider things final," she said. "There is still a way out if those responsible listen to the international community."
She said the Economic Community of West African States (ECOWAS) would hold a summit "probably on Sunday," where "possible sanctions could be decided," adding that France would support them.
France, which has 1,500 soldiers in Niger, previously called for "the restoration of the integrity of Nigerien democratic institutions."
But armed forces chief General Abdou Sidikou Issa has swung his weight behind the putschists.
"The military command... has decided to subscribe to the declaration made by the Defence and Security Forces... in order to avoid a deadly confrontation," he said in a statement.
The landlocked state is one of the world's poorest. Since gaining independence from France in 1960, it has experienced four coups as well as numerous other attempts -- including two previously against Bazoum.
The 63-year-old is one of a dwindling group of elected presidents and pro-Western leaders in the Sahel, where a jihadist insurgency has triggered coups in Mali and Burkina Faso.
Their juntas have forced out French troops, and in Mali the ruling military has woven a close alliance with Russia.
Coup leader Colonel Amadou Abdramane appeared on national television Thursday urging "the population to remain calm," after a group peeled off from a gathering in support of the putschists in Niamey and ransacked the headquarters of Bazoum's PNDS party, setting fire to vehicles in the car park.
About 1,000 people, mostly youngsters, had turned out before the National Assembly in the capital as well as several hundred in the town of Dosso, AFP journalists saw.
Some held Russian flags and chanted anti-French and pro-Moscow slogans.
"We want the same thing as in Mali and Burkina Faso," shouted 19-year-old student Alassane Alhousseini.
"We want to take our destiny in our own hands."
Niger's coup leaders -- 10 men in military uniform -- had appeared on television overnight.
Colonel Abdramane announced they were taking power following "the continued deterioration of the security situation, poor economic and social governance."
Under the banner of the Defence and Security Forces (FDS), they had "decided to put an end to the regime," and all institutions were being suspended, the borders closed and a night-time curfew imposed.
The president of neighbouring Benin, Patrice Talon, had been expected in the capital to mediate, but there was no further news of the mission.
ECOWAS on Thursday demanded Bazoum's "immediate release," saying he "remains the legitimate and legal President of Niger."
The latest coup effort raises fears of further instability and a "risk of disengagement" on the security front, said Alain Antil, head of the Sub-Saharan Africa Centre at France's International Relations Institute (IFRI).
"Bazoum was very fragile. His election was contested by the opposition... and it was known that a certain number of army officers were fairly unhappy with the situation," said Antil.
UN Secretary-General Antonio Guterres voiced alarm at instability in the Sahel, saying he was "extremely worried" about the extremism and military upheaval.
UN humanitarian operations have been suspended in Niger following the coup, a spokesman said.
The number of people in need of humanitarian assistance in the country climbed from 1.9 million in 2017 to 4.3 million in 2023, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).
US Secretary of State Antony Blinken said he had spoken to Bazoum to offer support from Washington, which deploys about 1,100 troops in Niger.
Russia -- isolated since invading Ukraine in February 2022 -- joined nations appealing for Bazoum's release and peaceful dialogue.
Poverty and jihadism
Bazoum took office after elections two years ago, in Niger's first peaceful transition since independence.
He had been interior minister and right-hand man to former president Mahamadou Issoufou, who voluntarily stepped down after two terms.
The country of 22 million is two-thirds desert and frequently ranks at the bottom of the UN's Human Development Index.
Niger faces two jihadist campaigns -- one in the southwest, which swept in from Mali in 2015, and the other in the southeast, involving jihadists from northeastern Nigeria.