Two explosions at a petrochemical plant in southern France may have been the result of a “malicious act,” sources close to the case say.

The sources, who made the remarks on the condition of anonymity, said the two blasts occurred at a plant operated by the petrochemical company LyondellBasell in the Provence-Alpes-Côte d'Azur region's Berre-l'Etang commune, during the early hours of Tuesday.

"The probability that these two fires in tanks 500 meters apart could be accidental is very low. Investigators are leaning towards the theory of a voluntary act," one of the sources said.

A second source said the blasts were most likely not accidental and "a malicious act is a serious hypothesis."

The blasts reportedly caused fire in two tanks, which contained petrol and naphtha, a flammable liquid.

No injuries were reported. French authorities say they have launched an investigation into the incident.

Following the explosions, a large cloud of black smoke was visible above the plant from far away.

More than 120 firemen and 50 vehicles were sent to the area near Marseille-Marignane Airport.