Baton Rouge, Louisiana - At least one person died and power outages hit more than one million customers in the United States as Hurricane Ida hit the country's south on Sunday.

The state of Louisiana, where Ida made landfall as a Category 4 storm at midday, has been hit hardest, according to the website poweroutage.us.

Around 996,000 households lost electricity in the state, while some 36,000 customers were affected in neighboring Mississippi.

All of New Orleans lost power, the city's emergency preparedness campaign (NOLA Ready) said, citing utility firm Entergy.

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"The only power in the city is coming from generators," NOLA Ready tweeted. Nearly 400,000 people live in New Orleans.

Entergy said the hurricane damaged all eight lines that supply the city. Repair work is ongoing but power is not expected to be restored quickly, the supplier said.

The first storm-related death was reported in Prairieville, south of the city of Baton Rouge, Louisiana's state capital. The local sheriff's office said on Facebook the victim was killed Sunday evening by a falling tree.

The National Hurricane Center (NHC) had warned earlier that a hurricane of such strength usually causes catastrophic destruction on land.

Photos and videos showing massive flooding and damage from the hurricane, while local media reported homes were destroyed, streets flooded, and trees and utility poles downed.

The storm ripped off parts of a hospital roof in the town of Galliano. No one was injured.

While it has been downgraded to Category 2 storm, gusts were still reaching speeds of over 100 miles per hour as of late Sunday, according to the NHC.

Ida is expected to keep losing strength as its center moves towards the city of Baton Rouge, but it's still likely to be at least a Category 1.

The storm comes 16 years to the day since the state was battered by Hurricane Katrina, which caused the deaths of more than 1,800 people, alongside catastrophic damage and flooding in and around New Orleans.

Louisiana Governor John Bel Edwards told CNN Ida would be "the most severe test" of New Orleans' storm defenses, but said he was optimistic the system would hold.

The city has made massive investments in flood protection since Hurricane Katrina hit exactly 16 years ago.

Edwards said he was more concerned about the areas south-west of the city that do not have as developed flood protection, where many of the residents had already left their homes and headed for safety.

Several districts imposed a curfew to ensure residents who haven't already left will stay in their homes in the face of extreme wind gusts and possible flooding. East Baton Rouge, where 440,000 people live, was under curfew from Sunday night to Monday morning.

The first serious signs of the storm system reached the coastal south-west of New Orleans at noon on Sunday, with local media broadcasting footage of flooding and violent gusts of wind.

By midday, the NHC had already measured a rise of over 6 feet in the water level in a coastal wetland south of New Orleans. At the city's airport, wind gusts had reached more than 60 miles per hour.

In advance of the storm's landfall, President Joe Biden authorized emergency measures for Louisiana and neighboring Mississippi.

"To the people of the Gulf Coast: Please follow the instructions of local officials during this dangerous time," Biden later tweeted. "As soon as Hurricane Ida passes, we will put this country's full might behind your rescue and recovery."