Thick black smoke billowed over the northern French town of Rouen on Thursday after a spectacular fire broke out at a chemical factory, leading authorities to close schools and warn about possible pollution of the nearby Seine river.Two hundred firefighters and 60 fire vehicles were battling the blaze that erupted at around 02:30 at a storage facility owned by a manufacturer of industrial lubricants and fuel additives."Let's not panic about this situation, but we need to be very careful," Interior Minister Christophe Castaner told RTL radio, adding that "there is nothing to make us believe there is a risk from the smoke".Schools and creches nearby would remain shut, he said, while local authorities have asked people living near the factory to stay at home and everyone else to avoid unnecessary journeys."The smoke is really striking and wherever you are in Rouen you can see it," Marina Andre, a 25-year-old who works in a bar near the factory, told AFP."You can smell fuel, not really a burned smell, it's very distinctive," she said, adding that people were still going to work and even cycling near the site of the blaze on the bank of the Seine river.Environmental threatThe factory belongs to US multinational Lubrizol, which is owned by the billionaire American investor Warren Buffett."We're still fighting the blaze, with the risk that pollution could spill into the Seine," a top regional official, Pierre-Andre Durand, told reporters, adding that the danger was an overflow of fuel or polluted wastewater into the river.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.And 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, which is famed for its cathedral.Firefighters stated shortly before midday that "the fire is contained, not out".They said they were concentrating on trying to prevent it from spreading to other parts of the factory, which is just a few kilometres from the centre of the city of around 100 000 people."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," Jean-Yves Lagalle, the head of firefighting in the region, told reporters.