Three young men, including a former naval signalman, were being held Thursday by France's intelligence services, accused of plotting to kidnap and decapitate a member of the armed forces at a military base.

Interior Minister Bernard Cazeneuve said security forces staged dawn raids on Monday and arrested four people, aged between 16 and 23, who were "planning to commit a terrorist act" at a French military installation.

The youngest was quickly released but the other three are suspected of planning to kidnap and behead a member of the military on film, possibly on December 31 when the facility was thinly staffed.

The oldest of the group served as a navy signalman at the base around the southern town of Collioure, which is also used for training by elite commando forces.

Identified only as Djebril, he was discharged from the navy in January 2014 for back problems, said a source close to the investigation, and the target is thought to have been his former boss.

The other key plotter was just 17, and was already being closely watched by authorities due to his activities on social media and connections to French jihadists in prison.

All three had been planning to travel to jihadist-controlled areas of Syria, the security source said, but the 17-year-old's mother became concerned about his radicalisation and contacted the authorities.

He was interviewed by counter-terrorism officials and was aware he was under surveillance.

"They claim to be part of Daesh," said a source close to the investigation, using an alternative Arabic acronym to refer to the jihadist Islamic State group that controls swathes of Syria and Iraq.

No weapons were found during the arrests, the source said, although officials discovered documents on preparing explosives.

The news of the arrests followed a statement from President Francois Hollande, who said attacks had been thwarted in recent days.

"This week, we stopped terrorist attacks which could have taken place," Hollande said on a visit to the southern city of Marseille.

France remains on high alert more than six months after jihadist attacks in January that claimed 17 lives and started with shootings at satirical magazine Charlie Hebdo.

The suspected plotters are now in the custody of France's intelligence services, the DGSI, and anti-terrorism prosecutors in Paris have opened a probe.

"We are facing a terrorist threat that we have never seen before -- an external threat and an internal threat," said Prime Minister Manuel Valls.

- Jihadist 'hotline' -

News of the foiled attack came a day after two blasts on Tuesday at a petrochemical plant near Marseille, described by Cazeneuve as a "criminal act".

Investigators had yet to pin down a motive for the explosions and there was currently "no link" with the foiled attack on the military base, he added.

The government says there are 1,850 French citizens or people living in France who are "implicated" in jihadist networks, with around 500 in Syria or Iraq.

France, which is home to Europe's largest Muslim population, has beefed up security, posting 30,000 police officers and soldiers outside 5,000 sites deemed sensitive such as schools and places of worship.

Authorities have also set up a hotline for friends or family concerned that someone could be tempted to wage jihad -- an effort that has yielded 2,500 leads.

Following controversial "anti-terror" laws passed last year, France is also preventing suspected jihadists from leaving the country -- some 118 travel bans have been enforced since the legislation was passed in November.

Cazeneuve said 29 people had been prevented from entering the country in that time.

A further 40 "preachers of hate, including pseudo-, self-proclaimed imams" had been kicked out of France.

Paris has also tightened security around sensitive sites such as factories, calling for "maximum vigilance", although experts have warned it is extremely difficult to defend against attacks on such locations.

Last month, a man with a suspected link to IS spiked his boss's severed head onto the fence of a US-owned gas factory in eastern France.

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