A soot-covered corn cob in Saint Martin du Vivier, near Rouen in northern France. Farmers in the area have been ordered to destroy crops tainted by the Lubrizol blaze. — AFP Photo

ROUEN — French officials faced growing pressure on Tuesday over their response to pollution concerns following a massive fire at a chemicals plant, which spewed huge clouds of smoke and soot across the northern city of Rouen.

Prime Minister Edouard Philippe told lawmakers that authorities would soon unveil a complete list of what burned at the Lubrizol factory in Rouen, which makes industrial lubricants and fuel additives.

He vowed "complete transparency" for residents worried by the acrid smell still permeating Rouen since the blaze last Thursday, saying the results of air, soil and water testing would be made public "as soon as they come in".

The Lubrizol plant, which is owned by US billionaire investor Warren Buffett, is situated just a few kilometres from the centre of Rouen, a city of some 100,000 people.

The smoke from the inferno spread 22km from the site.

The security chief for the Normandy region, Pierre-Andre Durand, told a press conference on Tuesday that tests conducted a day after the fire showed that fears of asbestos fibres from the plant's destroyed roof contaminating the air had proven "unfounded".

He added that preparations were being made to remove 160 barrels of chemicals in a "delicate state" from the state.

Philippe visited the factory on Monday night in a bid to reassure people on the contamination risks.

But while making reassuring noises the authorities have also banned the harvesting of crops or the sale of animal products from a wide swathe of countryside surrounding the plant, telling farmers than any exposed produce would have to be destroyed.

The decision affects more than 1,800 farmers whose fields were tainted by the oily soot, though the government promised quick compensation for their losses.

Several hundred people marched through Rouen on Tuesday evening to demand "the truth" about the impact of the fire on public health.

"They're hiding the truth, it's up to us to investigate," and "Our children in danger" read some of the placards waved by protesters.

At least five schools in Rouen either closed or told parents to come and fetch their children on Tuesday over the fallout from the fire.

One school reported an "unpleasant smell" and said students had been "complaining of nausea and itchy throats and eyes."

School officials were unavailable for comment.

‘Not a conspiracy theorist'

A public protest was called for Tuesday evening to demand that officials disclose all they know about the smoke and soot that settled across Rouen and the surrounding areas.

Corinne Lepage, a former environment minister and lawyer who filed a lawsuit on Monday on behalf of the Respire ("Breathe") advocacy group, said she believed the state was not doing enough to protect residents.

"It seems to me that the necessary analyses haven't been carried out, and that we're dealing with products they don't want to talk about, like dioxins, asbestos and heavy metals," she said.

A local police union has also demanded an emergency meeting with superiors about the potential exposure risks for officers who fought the blaze.

Many Rouen residents were still wearing face masks as they went about the city on Tuesday.

Older people and those with health problems had been warned to stay inside in the days following the fire.

Officials also deployed berms along the Seine river, which runs through the city, to try to prevent inky black deposits on the water from flowing downstream toward the Channel at Le Havre, a major fishing harbour.

The cause of the blaze is not yet known. — AFP