Two explosions have occurred at the flood-hit Arkema SA chemical plant in Crosby, Texas, and there is a risk of further blasts.
The Arkema plant makes organic peroxides used in the production of plastic resins, polystyrene, paints and other products.
"Organic peroxides are extremely flammable and, as agreed with public officials, the best course of action is to let the fire burn itself out," the company said.
"We want local residents to be aware that the product is stored in multiple locations on the site, and a threat of additional explosion remains.
"Please do not return to the area within the evacuation zone until local emergency response authorities announce it is safe to do so."
The sheriff said one of its deputies was taken to hospital after inhaling fumes and nine other people drove themselves to hospital as a precaution.
The flooded facility had lost power and backup generators, leaving it without refrigeration for chemicals that become volatile as the temperature rises.
The company evacuated remaining workers at the damaged plant on Tuesday, and Harris County ordered the evacuation of residents within a 2.4-km radius due to the risk of an explosion and subsequent leak.
Tropical Storm Harvey has killed at least 35 people and forced 32,000 people into shelters since coming ashore on Friday near Rockport, Texas, on the Gulf of Mexico Coast.
It has caused devastating flooding in Houston and other parts of Texas, and is the most powerful hurricane to hit the state in half a century.
Texas governor Greg Abbott said on Wednesday that floodwaters would linger for up to a week and said the area affected was larger than that hit by 2005's Hurricane Katrina, which killed more than 1,800 people in New Orleans, and 2012's Superstorm Sandy, which killed 132 around New York and New Jersey.
Harvey is predicted to be one of the most expensive natural disasters in US history.