PARIS (AFP)--A thunder storm hit a Total SA (TOT) petrochemicals facility in eastern France two days before a blast that killed two workers and injured six others, the group's head of safety said Thursday.

The deadly accident took place Wednesday at the plant in Carling near the German border, when a steam cracking furnace, which converts oil distillates into petrochemicals, blew up for an unknown reason.

Total says the blast occurred inside a steam production unit as the machine was being fired back up after a stoppage linked to the bad weather.

"There was an electrical incident two days before, as a result of serious weather disturbances, storms on the site at Carling," Jean-Marc Jaubert told RTL radio.

Jaubert denied the blast could have been caused by firing the oven up too fast. "We do it unit by unit, we put everything progressively back in service, check it is all working properly, before moving to the next unit," he said.

But the CGT union, which represents many of the workers at the plant, alleged that "the accident would not have happened" if the cracker had been equipped with an automated start-up system.

"This accident occurred at an industrial site heavily impacted by Total's financial decisions, which require a restructuring with job cuts and destabilise work teams," it said.

Total's safety chief said the group would cooperate fully with the French police investigation to determine the cause of the blast, and would step up surveillance of its other steam-cracking facilities.

Unions at the plant called on the energy giant to show full transparency, as the CGT said it feared "Total will do all it can to hide its responsibility," saying the group was "already blaming the weather."

The CGT said it was the latest in a string of fatal accidents in the group's French operations, including the death of a worker by intoxication at Total Provence and the death of another in a fuel tank blast at Total Flandres.

A 21-year-old worker was killed by the force of the blast, and a second, aged 26, died buried in rubble. Two of the six injured were hospitalized, while the four others were discharged in the evening with minor injuries.

French officials have ruled out any risk of pollution from the blast, saying only steam escaped from the cracker.

The blast was the third serious explosion at a Total plant this decade.

Operated by Total chemicals subsidiary TPF, the Carling plant produces a range of petrochemicals including ethylene, propylene, methane and styrene, and plastic consumer products such polyethylene and polystyrene.

The products are produced by the process of steam cracking, whereby petroleum and either gas oil or naphtha are mixed in a cracking oven with superheated steam at 800 degrees Celsius.