Hurricane Ida has pounded Louisiana after sweeping ashore from the Gulf of Mexico, flooding wide areas under heavy surf and torrential rains as fierce winds toppled trees and power lines, plunging New Orleans into darkness.
The storm, though greatly diminished as it churned inland toward western Mississippi early on Monday, was expected to continue unleashing heavy downpours "likely to result in life-threatening" flooding, the National Hurricane Centre said.
On Sunday night, the sheriff's office in Ascension Parish reported the first known US fatality from the storm, a 60-year-old man killed by a tree falling on his home near Baton Rouge, the state capital.
Ida, the first major hurricane to strike the United States this year, made landfall around noon on Sunday as a ferocious Category 4 storm over Port Fourchon, a hub of the Gulf's offshore oil industry, packing sustained winds of up to 240km/h.
Its arrival came 16 years to the day after Hurricane Katrina, one of the most catastrophic and deadly US storms on record, struck the Gulf Coast.
President Joe Biden declared a major disaster in the state, ordering federal assistance to bolster recovery efforts in more than two-dozen storm-stricken parishes. The full extent of storm damage remained to be seen at daybreak.
Within 12 hours of landfall, Ida had weakened into a Category 1 hurricane on the five-level Saffir-Simpson scale, with top winds clocked at 135km/h as the storm pushed about 160km inland past New Orleans, Louisiana's largest city, early on Monday.
By then, Ida had ploughed a destructive path that submerged much of the state's coastline under metres of surf, with flash flooding reported by the National Hurricane Centre across southeastern Louisiana.
Nearly all offshore Gulf oil production was suspended in advance of the storm, and major ports along the Louisiana and Mississippi coasts were closed to shipping.
Power was knocked out Sunday night to the entire New Orleans metropolitan area following the failure of all eight transmission lines that deliver electricity to the city, the utility company Entergy Louisiana reported.
One transmission tower collapsed into the Mississippi River.
More than 1 million Louisiana homes and businesses in all were without electricity by late Sunday night, according to the tracking site Poweroutage.US.
Residents of the most vulnerable coastal areas were ordered to evacuate days ahead of the storm.
Those riding out the storm in their homes in New Orleans braced for the toughest test yet of major upgrades to a levee system constructed following devastating floods in 2005 from Katrina, a hurricane that claimed some 1800 lives.
The US Army Corps of Engineers said the newly reinforced New Orleans levees were expected to hold, though they said they said the flood walls could be overtopped in some places.
Videos posted on social media showed storm surge flooding had transformed sections of Highway 90 along the Louisiana and Mississippi coast into a choppy river.