A major fire at a chemical plant in Rouen, France broke out in the early hours of Thursday, leading authorities to close schools and tell thousands of people nearby to stay in their homes. 

Operating behind a security cordon, around 200 firefighters used more than 50 fire vehicles to tackle the blaze, which started at around 02:40 local time. As dawn broke, huge black clouds were billowing across the French port city on the River Seine. 

While authorities were quick to note there had been no reported victims, they still urged caution. "There is nothing to make us believe there is a risk from the smoke," said France's interior minister Christophe Castaner. "Let's not panic about this situation, but we need to be very careful."

Castaner said schools and creches near the factory would remain closed, while local authorities recommended that people living nearby should stay at home and everyone else should avoid unnecessary journeys. The capital of Normandy in northern France, Rouen has a population of approximately 650,000 in its metropolitan area. 

The factory belongs to Lubrizol, a US-headquartered company owned by billionaire investor Warren Buffett. His Berkshire Hathaway investment firm bought the company in 2011 for $9 billion. Lubrizol's French subsidiary makes additives for engine oils, fluids and additives for industrial lubricants and additives for gasoline and diesel fuel.

It is not the first time the Lubrizol factory has had problems. In January 2013, it leaked the gas mercaptan, which has an odor like rotten eggs. The smell drifted as far as Paris and Southern England. While in 2015, 2,000 liters of mineral oil found their way into Rouen's sewer system.