Seven victims were taken to Baton Rouge General Medical Center and more were expected, the hospital said on its website.
Workers scrambled over gates and ran from the plant when they heard the explosion, which shook the ground and was followed by a "ball of fire," Daniel Cuthbertson, a fuel truck operator with CBI construction based in Taft, Calif., said later at a police roadblock.
The Williams Companies said in a news release on its website that the flow of chemicals to the fire had been cut off by 10 a.m.
State Police Capt. Doug Cain said the explosion occurred around 8:30 a.m. at the plant, which makes ethylene and propylene—highly flammable gases that are basic building blocks in the petrochemical industry.
A thick plume of black smoke rose from the plant. At a roadblock several miles away, where family members waited anxiously to hear about loved ones, flames were easily visible above the trees.
Kelly said an ethylene fire at ground level reportedly was waning several hours after the explosion.
Southern Louisiana is home to a large share of the country's petrochemical facilities and has seen at least two other blasts in the past two years.
Pressure on the industry to improve safety has increased since a blast at the Texas City refinery killed 15 people in 2005, among the worst such industrial accidents in decades.
A blast last month at a fertilizer plant in West, Texas, that killed 14 people has also sharpened attention on handling of volatile chemicals.
—By The Associated Press and Reuters.