At least seven people have been killed after a “super fog” of smoke from marsh fires and dense morning fog caused a series of crashes near New Orleans in the US that left a long stretch of mangled and scorched vehicles.

An estimated 158 vehicles were involved and 25 people were injured in the incident on Interstate 55, according to the Louisiana State Police.

They warned the death toll could climb as first responders worked into the night looking for victims.

Vehicles were crushed, piled on top of each other and engulfed by flames.

Some people got out of their vehicles and stood on the side of the road or on the roof of their cars looking in disbelief at the disaster, while others cried out for help.

Clarencia Patterson Reed was driving with her wife and niece and could see people waving their hands for her to stop, but when she did her car was hit from behind and on the side by two other vehicles, she told The Times-Picayune/The New Orleans Advocate.

“It was ‘Boom. Boom’. All you kept hearing was crashing,” Ms Reed said. She was able to scramble out of her car, but her wife was pinned inside and injured her leg and side.

Another driver, Christopher Coll, said he was already braking when a pick-up truck “drove up on top of my work trailer and took me for a ride”.

Mr Coll could smell smoke as he heard the sounds of crashing cars and popping tyres. He was able to kick open his passenger door to escape and then helped others — pulling out one person through a car window.

While 25 people were transported to hospital, with injuries ranging from minor to critical, others sought medical aid on their own, authorities said.

Governor John Bel Edwards asked for prayers “for those hurt and killed” and issued a call for blood donors to replenish dwindling supplies.

Louisiana State Police shared aerial photos on their Facebook page showing the crashed cars and extensive debris on both northbound and southbound lanes of the elevated interstate, which passes over swamp and open water between lakes Pontchartrain and Maurepas.

As of Monday afternoon, state troopers were still working “to notify families, investigate the exact causes of the crashes” and co-ordinate with the state’s transportation department to have the bridge inspected.

The National Weather Service said there were multiple wetland fires in the region. Smoke from the fires mixed with fog to create a “super fog”.