The explosion took place at Williams Chemical Plant in Ascension Parish, Louisiana and a fire after the explosion appeared to be contained in the plant.
A local fire department said it was treating a number of people injured in the blast.
Images on the Times - Picayune news website showed a large ball of flame and a thick column of smoke coming from the plant.
NBC News reported 25 injuries, citing Paige Hargrove, executive director of the Louisiana Emergency Response Network.
Six burn victims were taken to hospitals by helicopter, WAFB television reported, citing the Iberville Office of Emergency Preparedness.
Corey Gautreaux, captain of City of Gonzales Fire Department, said emergency responders were treating people at the scene. There were no immediate reports of fatalities.
" It ' s an active scene. The fire department, the sheriff ' s office and hazmat(hazardous materials) team are responding to the explosion at the Williams Olefins plant, " said Amy Johnson, a spokeswoman for the Ascension Parish Office of Homeland Security and Emergency Preparedness.
Authorities ordered people within a 2 - mile(3 - km) radius to remain in their homes, in part because of the smoke, said Lester Kenyon, a spokesman for Ascension Parish. Local media reported roadways in the area had been closed.
Ship traffic on river remained unaffected, the US Coast Guard said.
The plant produces approximately 1.3 billion pounds of ethylene and 90 million pounds of polymer grade propylene, according to the Williams website.
These chemicals are the basic building blocks in the petrochemical process used to make plastics.
Williams operates the plant and holds an 83 percent ownership interest in the Geismar facility, the company ' s website said.
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.