The two suspected double-agents were charged last December, but the incident only came to light late on Thursday following French media reports and a statement from Defence Minister Florence Parly.

"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."

A security source, speaking to AFP on condition of anonymity, said that the men were suspected of working for China.

At least one of them had been posted in the country, French television show Quotidien on the TMC channel reported, adding that the conspiracy had been uncovered by French counter-intelligence.

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

Concerns about Chinese spying in Australia led to reforms to espionage and foreign interference legislation last year which singled out Beijing as a focus of attention.

The country has long been accused of commercial spying to capture valuable intellectual property, but its national security operations are also thought to have grown in step with its expanding foreign activity.

Asked Friday about the arrests in France, China's foreign ministry said it was unaware of the situation.

- 'Extremely vigilant' -

The arrested French agents are now retired but Parly said they were "quite likely" still in service at the time.

Investigators were still determining how long they had been passing along intelligence, she said.

She also declined to specify the nature of the 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 constraints pending trial, according to the same source.

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.

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