Two blasts at a petrochemical plant in France appear to have been a "criminal act" but investigators have been unable to pin down a motive, Interior Minister Bernard Cazeneuve said Wednesday.

"There is a probe underway. The first indications show that we are dealing with a criminal act, but no motive has been established," Cazeneuve told the lower house of parliament.

He said prosecutors would provide further details later Wednesday.

A source close to the investigation said that officials earlier Wednesday had discovered a device that could have been used to start a fire.

The twin explosions happened at a plant in the small town of Berre-l'Etang near the southern city of Marseille in the early hours of Tuesday 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.

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," said Prudhon.