Laura's powerful gusts uprooted trees and four people were crushed to death in separate incidents of trees falling on homes. The state's department of health said late on Thursday that two more fatalities were attributed to the hurricane - a man who drowned while aboard a sinking boat and a man who had carbon monoxide poisoning caused by a generator in his home.

In Westlake, a chemical plant caught fire when hit by Laura and the flames continued to send a chlorine-infused plume of smoke skyward nearly 24 hours after landfall.

Laura caused less mayhem than forecasts predicted but officials said it remained a dangerous storm and it would take days to assess the damage. At least 867,000 homes and businesses in Louisiana, Texas and Arkansas remained without power on Thursday afternoon.

"This was the most powerful storm to ever make landfall in Louisiana," Governor John Bel Edwards told a news conference. "It's continuing to cause damage and life-threatening conditions."

Laura's maximum sustained winds of 241km/h upon landfall easily bested Hurricane Katrina, which sparked deadly levee breaches in New Orleans in 2005, and arrived with wind speeds of 201km/h.

The NHC said Laura's eye had crossed into southern Arkansas late Thursday afternoon and was heading to the northeast at 24km/h. The storm could dump 178mm on portions of Arkansas, likely causing flash floods.

Laura was downgraded to a tropical depression at 10pm and the NHC said it would move to the mid-Mississippi Valley later on Friday and then to the mid-Atlantic states on Saturday.

Laura's howling winds levelled buildings across a wide swath of the state and a wall of water that was 4.6m high crashed into tiny Cameron, Louisiana, where the hurricane made landfall about 1am.

A calamitous 20-foot storm surge that had been forecast to move 64km inland was avoided when Laura tacked east just before landfall, Edwards said. That meant a mighty gush of water was not fully pushed up the Calcasieu Ship Channel, which would have given the storm surge an easy path far inland.

Tropical-force winds were felt in nearly every parish across Louisiana - and Edwards warned the death toll could climb as search and rescue missions increase.

Residents of Lake Charles heard Laura's winds and the sound of breaking glass as the storm passed through the city of 78,000 with winds of 137km/h and gusts up to 206km/h in the hour after landfall.

National Guard troops cleared debris from roads in Lake Charles on Thursday afternoon. There were downed power lines in streets around the city, and the winds tipped a few semi-trucks onto their sides.

Australian Associated Press