France foils terror plot to decapitate army officer

France has foiled a plan to attack the country's military, the interior minister said Wednesday, as a source close to the investigation said the suspects had been planning a beheading.

Interior Minister Bernard Cazeneuve told reporters that four accused who had been planning "a terrorist act against French military facilities" were in custody of the country's intelligence services, the DGSI.

Aged 16 to 23 and including a former member of the navy, they were arrested at dawn in different parts of the country, the minister said.

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 coastal city of Marseille.

Cazeneuve said no link had been established between the foiled assault and two blasts on Tuesday at a petrochemical plant near Marseille.

A source close to the investigation into the thwarted attack, who asked to remain anonymous, said the four people arrested had been planning to film the decapitation of high ranking member of the military.

One of four suspects, who was identified as the mastermind, had been planning to travel to jihadist-controlled areas of war-torn Syria, Cazeneuve said.

Earlier, a security source had told AFP that four suspects with an "Islamist profile" had been arrested on suspicion of planning an attack somewhere in France, without giving further details.

'Criminal' blasts

Separately, Cazeneuve told the lower house of parliament that the two blasts at a petrochemical plant on Tuesday appear to have been a "criminal act" but investigators had yet to pin down a motive.

Officials discovered devices thought to have started the twin explosions at the plant in the small town of Berre-l'Etang near Marseille in the early hours of Tuesday, which sparked huge fires but no one was hurt.

Two tanks full of petrol and naphtha -- a flammable liquid distilled from petroleum -- caught fire after the blasts and a thick cloud of black smoke was visible several kilometres away.

Prosecutors said an explosive device was found in a third tank, but had failed to trigger a major explosion.

France remains on edge after the Charlie Hebdo attacks in January where a trio of gunmen killed a total of 17 people starting with a massacre at the satirical magazine.

And last month, a man with suspected links to the Islamic State group spiked his boss's severed head onto the fence of a US-owned gas factory.

Paris has tightened security around sensitive sites such as factories, calling for "maximum vigilance".

But experts have warned it is extremely difficult to defend against attacks on such sensitive sites.

"There is no such thing as zero risk," said Philippe Prudhon, a technical expert at the UIC union of chemical industries.

"If someone really wants to cause harm, it will be difficult to stop him or her. We have to realise that we have been in a fundamentally different environment for the past three years," Prudhon said.