Residents are being warned to shelter in place and close doors and windows due to a toxic and "extremely dangerous" chemical leak in Louisiana after Hurricane Laura slammed into the Gulf Coast on Thursday.

State police were responding to a chlorine leak at a company that makes chemicals along Interstate 10 just west of Lake Charles, which was hard-hit when the category-4 storm made landfall.

The leak is at the BioLab chemical manufacturing facility in Westlake. Residents in the area are being told to close their doors and windows, turn off their air conditioning and stay inside.

The chemical spill has reportedly flowed over the I-10, the cross-country highway connecting the southern states from Florida to California.

Storm chaser, Jeff Piotrowski, posted on Twitter: "Chemical leak south side of I-10 west Lake area west of Lake Charles. 911 is aware they are overwhelmed."

Fox metereologist, Brad Sowder, tweeted that it was an "extremely dangerous situation".

A cloud can be seen over a plant in the video and pictures posted on social media.

Law enforcement are working with plant managers to try and contain the leak. BioLab’s Lake Charles plant was built in1979 and manufactures trichloroisocyanuric acid, chlorinating granules and other chemicals used in such household cleaners as Comet bleach scrub and pool chlorine powder.

Both trichloroisocyanuric acid and chlorine are potentially acutely toxic to people and animals if ingested or inhaled. Chlorine gas, which can appear in the air as a greenish yellow cloud, was used as a chemical weapon in World War I. It is a potent irritant to the eyes, throat and lungs.

Hurricane Laura has weakened to a tropical storm with maximum sustained winds of nearly 70 mph. Authorities say the storm is now 50 miles southeast of Shreveport.

Hurricane Laura crashed into the Gulf coastline in the early hours of Thursday, with sustained 150mph winds, after officials warned it would bring an "unsurvivable" storm surge as high as 20ft.

Authorities had urged residents on the coast to evacuate, but not everyone did before winds started buffeting trees, and thousands of homes and businesses lost power. So far one fatality has been reported.

Brett Geymann of Moss Bluff, just north of Lake Charles, told the Associated Press how the eye of the storm passed directly over them. He said his house survived but every other building, structure and tree on his property was gone.

He said "It looks like 1,000 tornadoes" came through, with some houses "totally gone".

Wires contributed to this report