© Rammb/NOAA/Nesdis/AFP/GettyA satellite image shows Hurricane Laura reaching the coasts of Louisana and Texas.
The National Hurricane Center said early on Thursday the storm had struck Cameron in Louisiana, close to the Texas border, with maximum sustained winds of 150mph.
Forecasters said the storm surge could be 6 metres (20ft) deep and unsurvivable.
Authorities had previously implored coastal residents of Texas and Louisiana to evacuate, but not everyone did before howling winds began buffeting trees back and forth in an area that was devastated by Hurricane Rita in 2005.
Social media footage showed torrents of rain rushing sideways past lampposts in Lake Charles, and streets covered with water closer to the coast.
With hours of violent weather ahead, officials said the extent of destruction would not be clear until daybreak, when search-and-rescue missions would begin.
The storm system arrived, drawing energy from the warm Gulf of Mexico and during high tide.
"It looks like its in full beast mode, which is not what you want to see if you are in its way," a hurricane researcher at the University of Miami hurricane, Brian McNoldy, said.
Officials said at least 150 people rejected pleas to leave and planned to weather the storm in everything from elevated homes to recreational vehicles in coastal Cameron Parish.
"It's a very sad situation," said Ashley Buller, the assistant director of emergency preparedness. "We did everything we could to encourage them to leave."
A category 4 hurricane can render wide areas uninhabitable for weeks or months and shut down power for just as long. The threat of such devastation posed a new disaster-relief challenge for a government already straining under the coronavirus pandemic.
The parts of Louisiana that were under evacuation orders included areas with high rates of Covid-19.
The Texas governor, Greg Abbott, and his Louisiana counterpart, John Bel Edwards, had feared the dire predictions were not resonating with the public, despite authorities putting more than 500,000 coastal residents under mandatory evacuation orders.
Hurricane warnings were issued from San Luis Pass in Texas to Intracoastal City in Louisiana, and reached inland for 200 miles. Storm surge warnings extended from Freeport in Texas to the mouth of the Mississippi River.
On Twitter, President Donald Trump urged coastal residents to heed officials.
Associated Press in Delcambre, Louisiana
Covid response undermining belief in government pronouncements? Or just the usual public response from those who have heard many overblown hurricane warnings?