REUTERS - A large explosion and fire hit the Williams Olefins chemical plant in Geismar, Louisiana, on Thursday, local authorities said, causing at least some injuries.

The local fire department said it was treating an unknown number of injured people after the blast, which occurred at a plant along the Mississippi River just south of Baton Rouge.

Online images showed a large plume of smoke at the plant.

Captain Corey Gautreaux 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.

Local media said the fire broke out at around 8.30am local time. Local roadways were closed off.

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 per cent ownership interest in the Geismar facility, the company's website said.

Shares in parent company Williams Cos fell by as much as 3 per cent in early trading after the reports.