Acrid smoke billowed over the northern French city of Rouen on Thursday after a fire broke out at a chemical factory, forcing authorities to close schools and warn of pollution risks for the Seine River.
About 200 firefighters had brought the blaze under control by Thursday afternoon, although officials said it could be days at least before it was fully extinguished.
"The fight they will have to wage to get the situation fully under control will take several days, maybe even weeks," Interior Minister Christophe Castaner said after arriving at the site.
The fire erupted about 2.30am at a warehouse owned by Lubrizol, a maker of industrial lubricants and fuel additives, which is owned by billionaire American investor Warren Buffett.
It is just a few kilometres from the centre of the city, which has a population of 100,000.
Mr Castaner said firefighters managed to remove the most dangerous products that could have led to other explosions at other nearby industrial sites.
But the smoke had spread 22 kilometres and contained "compounds which can be a health hazard," he said.
"There was soot all over the streets, pavements and cars," said Jean-Claude Bleuzen, a deputy mayor of Preaux, about 15km north-east of Rouen.
Schools and creches nearby were shut until Monday, although nearby residents who were moved to safety will be able to start returning on Thursday evening, said Pierre-Andre Durand, the top regional official.
"We're still advising people with health problems to stay in their homes until Friday evening," Mr Durand said.
Several people were wearing face masks around the city.
Prosecutors have opened an inquiry into the cause of the fire, which woke residents with a booming explosion.
"The smoke is really striking and wherever you are in Rouen you can see it," Marina Andre, 25, who works in a bar near the factory, told AFP.
"You can smell fuel – not really a burnt smell, it's very distinctive," Ms Andre said.
She said people were still going to work and even cycling near the site of the blaze on the bank of the Seine.
Lubrizol said the fire damaged a storage facility, a drumming warehouse and an administrative building, and that the cause was still unknown.
It advised residents not to eat fruit or vegetables from gardens without washing it thoroughly, and not to touch the soot or other particles on the ground.
Mr Durand said there was a risk that fuel or waste water from the site could make retainment ponds overflow and reach the Seine, and that floating anti-pollution booms had been placed on the river.
"There is no pollution downstream," he said.
Mr Durand said that while some could escape as the tide came in, it would be easy to recover because it would be on the surface.
The Seine, one of France's biggest rivers, flows through Paris to the south and empties into the English Channel further north in Le Havre, which is home to a major fishing fleet.
In January 2013, the Lubrizol factory was responsible for a giant leak of the gas mercaptan, which smells like cabbage or rotten eggs and is often added to natural gas to alert people in case of leaks.
A cloud of the gas blew all the way to Paris and across the Channel into southern England, where residents complained about the odour.
In 2015, 2,000 litres of mineral oil, which is used in lubricants, leaked from the Lubrizol site into the local sewer system in Rouen.
"The biggest priority is to protect the most risky parts of the factory, which could explode and then create a domino effect of other fires," said Jean-Yves Lagalle, the head of firefighting in the region.
