A man is missing after being attacked by a massive alligator while walking through Hurricane Ida floodwaters in New Orleans, a Louisiana sheriff has said.

The 71-year-old man’s wife told sheriff’s deputies that she heard a commotion at around noon on Monday (local time), then walked outside to see the gator attacking her husband in the New Orleans suburb of Slidell, the St Tammany Parish Sheriff’s Office said in a statement.

“Once the attack stopped, she pulled her husband out of the floodwaters, and returned inside to gather first aid supplies,” the sheriff’s office said.

When the man’s wife realised how severely her husband had been injured, she got into her canoe and went to higher ground – nearly two kilometres mile away – to get help.

But when she returned, her husband was no longer lying on the steps of their home, the sheriff’s office said.

The sheriff’s office said it used high-water vehicles and boats in an attempt to find the man “but all attempts have been futile”. His name has not been released.

The sheriff’s office has warned New Orleans residents to be on guard while walking in flooded areas as the deadly hurricane has also displaced wildlife.

The death toll from Hurricane Ida, which hit Louisiana and Mississippi earlier in the week, is at least four. It includes two people killed on Monday night when seven vehicles plunged into a six-metre hole near Lucedale, Mississippi, where a road had collapsed.

Hundreds of thousands of Louisianans have also sweltered in the aftermath of the storm with no electricity, no tap water, precious little petrol – and no clear idea of when things might improve.

Long lines formed at the few service stations that had fuel and generator power to pump it. Neighbours shared generators and borrowed buckets of swimming pool water to bathe or to flush toilets.

“We have a lot of work ahead of us and no one is under the illusion that this is going to be a short process,” Governor John Bel Edwards said on Tuesday as the clean-up and rebuilding began in the late-summer heat.

New Orleans officials announced seven places in the city where people could get a meal and sit in air-conditioning. The city would have drive-through food, water and ice distribution locations set up on Wednesday, Mayor LaToya Cantrell said.

Ms Cantrell also ordered a curfew on Tuesday, calling it an effort to prevent crime after Hurricane Ida devastated the power system and left the city in darkness.

The mayor also said she expected the main power company Entergy to provide some electricity by Wednesday night, but stressed that did not mean a quick citywide restoration.

Mr Edwards said state officials also were working to set up food, water and ice distribution, but it would not start on Tuesday.

More than one million homes and businesses in Louisiana and Mississippi – including all New Orleans – were left without power when Ida slammed the electric grid on Sunday with its 240km/h winds, toppling a major transmission tower and knocking out thousands of kilometres of lines.

Officials said it could take weeks to restore electricity.

With treatment plants overwhelmed by floods or crippled by blackouts, some places faced shortages of drinking water. About 441,000 people had no water, and another 319,000 were under boil-water advisories.

In New Orleans, drivers lined up waiting to get into a Costco for petrol. Renell Debose spent a week in the New Orleans Superdome after 2005’s Hurricane Katrina, which killed 1800 people and left the city nearly uninhabitable.

She said she was willing to give it a few days without electricity, but no more than that.

“I love my city. I’m built for this. But I can’t make it without any air-conditioning,” she said.

-with AAP