French authorities are foiling new terror plots "every day", according to the country's prime minister.

Manuel Valls said France faced a threat from an estimated 15,000 homegrown radicals, warning that despite the efforts of security services, there "will be new attacks".

He was speaking to radio station Europe 1 as a 15-year-old boy was arrested in Paris on suspicion of planning an imminent knife attack.

"Today the threat is at a maximum, and we are a target," Valls said on Sunday.

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"Every day attacks are foiled ... (including) as we speak."


He warned that among the 15,000 people suspected of being in the process of being radicalised, 1,350 are under investigation.

"There will be new attacks, there will be innocent victims...this is also my role to tell this truth to the French people," he said.

The teenager detained in eastern Paris on Saturday had been under house arrest since April for suspected links to Islamic extremists following a search of his home.

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His arrest comes just days after a police dismantled a "terrorist cell" of female "commandos" in connection with an aborted attack near Paris' Notre Dame Cathedral.

Four women, the 15-year-old daughter of one of the women and a man were arrested last week after an abandoned car filled with gas canisters and diesel was found close to the landmark.

One of the women was charged with terrorism offences after telling police she and an accomplice had tried to set the vehicle alight but fled when they saw a man they believed to be a plain-clothes policeman.

Three of the women are accused of planning a separate attack.

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Paris prosecutor Francois Molins said they were acting on orders coming from Syria.

France is in a state of emergency following a series of Islamist attacks which have left 238 people dead since January 2015.

Islamic State has claimed responsibility for a number of the attacks including the Paris attacks in November in which 130 people were killed. It also claimed the man who killed 86 people with a truck in Nice in July was one it its "soldiers".

According to Interior Minister Bernard Cazeneuve, police have arrested 293 people this year for "links to terrorist networks".