New Orleans 'super fog' leaves at least seven dead and dozens injured in multiple pile-ups

At least seven people are dead and 30 injured after so-called "super fog" caused more than two dozen traffic pile-ups on a US highway.

Around 25 collisions happened on the I-55 west of New Orleans in southeastern Louisiana, St John the Baptist Parish Sheriff Mike Tregre, told CNN.

A mixture of dense fog and smoke from marsh fires combined to create the "super fog", which descended on Monday morning's rush hour, drastically lowering visibility for drivers.

Fires broke out in both the northbound and southbound carriageways between Ruddock and Manchac and traffic backed up for miles in both directions.

In the northbound lanes, about three 18-wheel trucks collided and were fully engulfed in flames, Sheriff Tregre, told TV station, WVUE.

In the southbound lanes, there were two reported multi-car pile-ups, one of which also caused a fire.

Videos taken by people on the highway showed how it had become a narrow junkyard of mangled cars and trucks, some of them burning.

One driver survived after his vehicle left an elevated section of the road, landing upside down in several feet of water below, WVUE said.

Mr Tregre said all first responders arrived on foot because the crashes left the area "completely gridlocked".

Rescue efforts, he said, "will take a while".

Parts of Interstates 55 and 10 west of New Orleans remained closed late on Monday morning and the 24-mile-long causeway over Lake Pontchartrain near New Orleans was closed at times.

The National Weather Service said on social media there were several wetland fires in the region and smoke from the fires mixed with fog to create a "super fog".

Visibility was expected to improve as the fog lifted, but it was unclear how long the marsh fires, smoke from which could be seen and smelled in the New Orleans area over the weekend, would continue to affect traffic.

Several local schools cancelled classes, the Times-Picayune/The New Orleans Advocate said.

The Blood Centre of New Orleans asked for blood donors in the wake of the accidents.