The letter, posted on the website of the rightwing Valeurs Actuelles magazine late Sunday, echoes the one published by the same publication last month. It too warned that a civil conflict was brewing.

Interior Minister Gerald Darmanin, a close ally of Macron, condemned the second letter as a "crude manoeuvre", accusing its anonymous signatories of lacking "courage".

The previous letter, signed by a handful of officers and some 20 semi-retired generals, sparked a furore. Prime Minister Jean Castex denounced it as unacceptable interference and France's top general vowed that those behind it would be punished.

It is not clear how many people are behind the current letter or what their ranks are.

In contrast to the previous one, it can be signed by the public, with Valeurs Actuelles saying more than 145,000 had done so by Monday afternoon.

"We are not talking about extending your mandates or conquering others. We are talking about the survival of our country, the survival of your country," said the letter, which was addressed to Macron and his cabinet.

The authors described themselves as active-duty soldiers from the younger generation of the military, a so-called "generation of fire" that had seen active service.

"They have offered up their lives to destroy the Islamism that you have made concessions to on our soil," they wrote.

They claimed also to have served in the Sentinelle security operation within France, launched after a wave of jihadist attacks in 2015.

They observed that for some religious communities "France means nothing but an object of sarcasm, contempt or even hatred".

It added: "If a civil war breaks out, the military will maintain order on its own soil... civil war is brewing in France and you know it perfectly well."

- 'Is this courage?' -

A high-ranking officer in military headquarters told AFP that the armed forces would not let the letter go without a response.

"A firm reminder will be made by the command on the respect of duty," said the officer, who asked not to be identified by name, adding that remaining apolitical was essential for maintaining the military's credibility.

"One can have personal convictions but the armed forces are apolitical and have absolute loyalty to the elected president. If you feel bad you can leave the army with a clean conscience," the officer said.

"I believe that when you are in the military you don't do this kind of thing in hiding," Darmanin told BFM television. "These people are anonymous. Is this courage? To be anonymous?"

The letter comes in a febrile political atmosphere ahead of 2022 elections, when Macron's main challenger is expected to again be the far-right leader Marine Le Pen.

Analysts say Macron has tacked to the right in recent months to prevent Le Pen from exploiting a series of attacks in late 2020 blamed on Islamist extremists who recently immigrated to France.

Prime Minister Jean Castex had labelled the rare intervention in politics by military figures in last month's letter "an initiative against all of our republican principles, of honour and the duty of the army".

France's armed forces chief of staff, General Francois Lecointre, said those who signed it would face punishments ranging from forced full retirement to disciplinary action.