Defence Minister Florence Parly, who oversees the country's General Directorate for External Security (DGSE), said Friday that she was not in a position to identify the country which recruited the agents, who were discovered and indicted in December.

"Two French agents in our service and probably one of the spouses of these agents are accused of serious acts likely to be considered acts of treason, on suspicions of delivering information to a foreign power," Parly told CNews television.

"I can't say much else," she added.

"France has partners but we live in a dangerous world, and unfortunately these types of things can happen."

French media reports, citing sources close to the inquiry, said China is suspected.

Parly said the agents were "quite likely" still in service at the time but investigators were still determining how long they had been passing along intelligence.

She also declined to specify the nature of compromised information, nor to reveal if the two agents were working together.

A judicial source told AFP late Thursday that two of the three suspects are being prosecuted for "delivering to a foreign power information that undermines the fundamental interests of the nation" and "compromising the secrecy of national defence".

"One of them has also been charged for direct incitement to the crime of treason," the source added.

The third person -- believed to be the wife -- has been indicted for "concealment of treasonable crimes" and placed under judicial control, meaning they are subject to certain constraints pending trial, according to the same source.

The armed forces ministry said: "These acts of extreme gravity have been detected by this service, which has brought these facts to its knowledge to the Paris prosecutor."

- 'Extremely vigilant' -

The revelations came just a few weeks after US authorities said they had indicted a former Central Intelligence Agency operative on charges of spying for China, following his arrest in January.

France's domestic intelligence service, the General Directorate for Internal Security, first discovered the suspected double agents, before alerting the defence ministry, which began its own investigations.

In a statement, the DGSE -- similar to Britain's MI6 or the United States's CIA -- said the revelations "are a major area of focus for the DGSE as well all French counter-intelligence services."

"We are being extremely vigilant in this matter," Parly told CNews.

The three people were charged on December 22, 2017, and two have been detained since then, the judicial source told AFP.

On Thursday, France's Quotidien television programme and Le Monde newspaper reported that four people were suspected of having been recruited by the Chinese authorities to spy on French foreign intelligence operations.

The defence ministry has refused to comment on the reports.