Tropical Storm Barry has buffeted the US state of Louisiana, bringing warnings of heavy rain and possible tornadoes even as it weakened.

After briefly becoming the first hurricane of the Atlantic season, Barry was downgraded to a tropical storm after making landfall yesterday. It nevertheless packed a serious punch as it moved inland, though there were few indications of widespread flooding.

Flights in and out of the airport in the state's biggest city New Orleans resumed today after all were cancelled a day earlier. Thousands of people had abandoned their homes, tens of thousands lost power and first responders were poised for action.

Fears that the levee system in New Orleans could be compromised eased after the Army Corps of Engineers voiced confidence that it would hold, but Mayor LaToya Cantrell urged residents not to be complacent.

"We are not in any way out of the woods," she said, adding that flash flooding could still occur into Sunday.

US President Donald Trump warned of "major flooding in large parts of Louisiana and all across the Gulf Coast.

"Please be very careful!" he said on Twitter.

Louisiana Governor John Bel Edwards said yesterday that the storm would intensify into today, with many areas seeing more rain overnight than they had during the day.

"Don't let your guard down thinking the worst is behind us," he told a press conference.

At 8am, the storm packed maximum sustained winds of 72km/h and was located southeast of Shreveport in western Louisiana, the National Hurricane Center said.

"Barry is forecast to weaken to a tropical depression later today," the NHC said.

Pete Gaynor, acting administrator of the Federal Emergency Management Agency, said "there are still life-threatening conditions that exist" as Barry moves north.

"The rain is the threat," he added, not only while it falls but in a couple of days when floodwaters move back down the Mississippi River to the Gulf of Mexico.

Mr Edwards said he had spoken to Mr Trump about the storm's impact.

"I thanked President Trump for his support and for approving our request for assistance," the governor said.

Tornadoes were possible in parts of Louisiana, Mississippi, western Alabama and eastern Arkansas, the NHC said.

Rainfall estimates had been lowered to between 15 and 30cm over south-central Louisiana but rivers and canals across the state's south were full.

The heavy winds scattered tree branches across roads and knocked over road signs, and in St. John's Parish next to New Orleans, local television footage showed some areas under 60cm of water.

The eye of the storm made landfall at Intracoastal City, a speck of a town with a few houses and businesses. Part of the main road was flooded yesterday afternoon, as were some waterfront businesses, with water rising by the minute.

News footage showed localised flooding, swollen waterways, and downed power lines and trees across south Louisiana. Rivers overtopped their levees in several locations, including part of coastal Terrebonne Parish, where authorities had issued a mandatory evacuation notice.

The Atchafalaya River swallowed the waterfront pedestrian promenade in Morgan City, which was entirely without power, as about 10 members of America's Cajun Navy citizen rescue group assembled under a highway overpass.

For many, the storm and potential for large-scale flooding revived unpleasant memories of deadly Hurricane Katrina.

Thousands packed up and left their homes as floodwaters hit low-lying areas like Plaquemines Parish, where road closures left some communities isolated.

Others hunkered down to ride out the squall, despite mandatory evacuation orders and the risk of dangerous storm surges.