Hurricane Laura tore through south-west Louisiana, destroying buildings in the city of Lake Charles and killing four people, including a 14-year-old girl, after making landfall early on Thursday.
It was one of the most powerful storms to hit the state.
The first fatality was reported in Leesville, Louisiana, when the girl died after a tree fell on her house, a spokeswoman for Governor John Bel Edwards said.
“We do expect that there could be more fatalities,” Christina Stephens said on Twitter.
The Category 4 storm prompted warnings of "unsurvivable" ocean surges and evacuation orders for hundreds of thousands of Gulf Coast residents before it reached the US state.
The storm passed through Lake Charles, with a population of 78,000, with winds of 137kph gusting to 206kph in the hour after landfall.
Hurricane Laura causes widespread damage on America's Gulf Coast
The windows of the city’s 22-floor Capital One Tower were blown out, street signs were toppled and pieces of wooden fence and debris from collapsed buildings lay scattered in the streets, footage on Twitter and Snapchat showed.
Satellite images revealed the immense size of the hurricane as it made landfall about 9am UAE time, while TV pictures showed heavy rain lashing the coastal city of Lake Charles and powerful wind uprooting trees.
Earlier the National Hurricane Centre warned that Laura could pummel the Louisiana and Texas coasts with an "unsurvivable storm surge" of up to six metres, and that tornadoes could form at the southern edges of the weather system.
"Take cover now. Treat these imminent extreme winds as if a tornado were approaching and move immediately to the safe room in your shelter," the centre said on Twitter.
"Take action now to protect your life."
While the worst projections were not realised, damaging winds and flooding rain would continue to spread inland later on Thursday, the centre said.
Hurricane Laura makes landfall in the US
Laura made landfall just before 1am in the small town of Cameron, Louisiana, the centre said.
It rapidly weakened to a Category 1 storm on Thursday morning with maximum sustained winds of 120kph), and was forecast to be downgraded to a tropical storm later in the day.
The centre warned that high water levels would persist along the Gulf Coast for hours as Laura moved north and then north-east.
The storm slammed the heart of the US oil industry, forcing oil rigs and refineries to shut down production.
Texas Governor Greg Abbott warned earlier that Laura's power was "unprecedented" and urged citizens to "get out of harm's way".
"Your property can be replaced," Mr Abbott said. "Your life cannot."
Vice President Mike Pence, speaking on the third night of the Republican National Convention, urged people in the storm's path to "heed state and local authorities".
The National Guard said it had more than 1,000 members in Texas to help with hurricane response.
Its assistance included 20 aircraft and more than 15 shelter teams.
In the Texas town of La Porte, near Houston, residents were stocking up on essentials and a voluntary evacuation was in place.
"I'm a little nervous but then I'm also like, 'OK, I know I'm going to be safe inside my apartment," Matthew Jones, 28, a security guard, told AFP.
"I got bread. I got lunch meat, peanut butter, jelly, gallons of water, snacks."
Mr Bel Edwards tweeted that parts of I-10, the interstate motorway that connects all of the southern US, were closed before Laura's arrival.
He urged those leaving to head north to drive around the closed section.
But the eastern coastal counties of Texas that had braced for the worst were largely spared Laura’s rage.
Galveston’s acting mayor, Craig Brown, told MSNBC that he was cancelling the island city’s evacuation on Thursday after the storm turned more towards Louisiana.
Becky Ames, the Mayor of Beaumont, Texas, said the situation was “manageable".
Mr Abbott was planning to travel to eastern Texas to survey the damage on Thursday, his spokesman said.
About 650,000 homes and businesses in Louisiana and Texas were without power early on Thursday, and local utilities in the storm’s path warned that would climb as the storm marched inland.
In New Orleans, devastated by Hurricane Katrina in 2005, the historic French Quarter was empty of tourists.
Sandbags were piled up in front of the doorways of colonial-style buildings and windows were boarded up with plywood.
The city remains traumatised from Katrina, which made landfall as a Category 3 storm, flooding 80 per cent of the city and killing more than 1,800 people.
Laura earlier caused flooding in Haiti and the Dominican Republic, killing at least 25 people.
The Atlantic storm season, which runs through November, could be one of the busiest ever this year, with the hurricane centre predicting as many as 25 major storms.
Laura is the 12th so far.
