President Emmanuel Macron said on Friday that France's envoy to Niger is living like a hostage in the French embassy and accused military rulers of blocking food deliveries to the mission.
The ambassador is living off "military rations", Macron told reporters in the French town of Semur-en-Auxois.
"As we speak, we have an ambassador and diplomatic staff who are literally being held hostage in the French embassy," he said.
"They are preventing food deliveries," he said, in an apparent reference to Niger's new military rulers. "He is eating military rations."
Niger's military leaders told French ambassador Sylvain Itte he had to leave the country after they overthrew President Mohamed Bazoum on July 26.
But a 48-hour ultimatum for him to leave, issued in August, passed with him still in place as the French government refused to comply, or to recognise the military regime as legitimate.
The coup has been condemned by France and most of Niger's neighbours.
Macron said the envoy "cannot go out, he is persona non grata and he is being refused food".
Asked whether France would consider bringing him home, Macron said: "I will do whatever we agree with President Bazoum because he is the legitimate authority and I speak with him every day."
Foreign Minister Catherine Colonna later said the ambassador "is working" and would stay at his post for as long as Paris wished.
"He is very useful for us with his contacts and those of his team," Colonna told LCI television, adding the ambassador still had a small team with him.
France keeps about 1,500 soldiers in Niger, and said earlier this month that any redeployment could only be negotiated with Bazoum.
The country's new leaders have torn up military cooperation agreements with France and asked the troops to leave quickly.
Macron has for weeks rejected the call to remove the French ambassador, a stance backed by the EU which has described the demand as "a provocation".
Like France, the EU "does not recognise" the authorities that seized power in Niger, said EU foreign affairs spokeswoman Nabila Massrali last month.
The impoverished Sahel region south of the Sahara has suffered what Macron has called an "epidemic" of coups in recent years, with military regimes replacing elected governments in Mali, Burkina Faso and Guinea as well as Niger.