Crews are working to clear the wreckage of more than 150 vehicles involved in a series of crashes near New Orleans caused by a “superfog” of marsh fire smoke and dense fog that killed at least seven people and injured another 25.

An estimated 158 vehicles were involved in the incident on Interstate 55 on Monday, according to Louisiana State Police, who warned on Monday night that the death toll could climb as first responders looked for victims.

The crashes began before 9am on Monday and there were several collisions along a one-mile span of the highway, Louisiana State Police Sergeant Kate Stegall said in a news briefing.

The highway was expected to remain closed at least until the bridge could be inspected for damage in daylight Tuesday, she added.

The crashes left a long stretch of mangled and scorched cars, trucks and lorries. 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 roofs of their cars looking in disbelief at the disaster, while others cried out for help.

Clarencia Patterson Reed, 46, was driving to Manchac with her wife and niece and saw people waving for her to stop, but once she stopped, two other vehicles hit her car from behind and the side, she told the Associated Press.

She escaped from her side of the car, but her wife was pinned inside with an injured leg and side. Others stepped in to help, Ms Patterson Reed said.

“I just thank God,” she said. “There was a casualty a few cars ahead of us.”

She said she could see the line of wrecked cars and hear cars continuing to collide.

“It was ‘Boom. Boom’. All you kept hearing was crashing,” she told The Times-Picayune/New Orleans Advocate.

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”.

He said he 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 taken to hospital, with injuries ranging from minor to critical, others sought medical aid on their own, authorities said.

State 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 northbound and southbound lanes of the elevated motorway, which passes over swamp and open water between Lake Pontchartrain and Lake 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 force said.

Traffic backed up for miles in both directions on I-55. The lack of visibility also prompted closures of parts of Interstate 10 and the 24-mile Lake Pontchartrain Causeway at times.

School buses were summoned to transport stranded motorists from the accident sites. At midday, state police told reporters at the scene that one vehicle had gone over the barrier and into the water, but the driver escaped unharmed.

The National Weather Service warned that a dense fog advisory would be in effect into Tuesday morning for parishes near lakes Pontchartrain and Maurepas.