Hurricane Laura has become a Category 4 storm and is expected to cause "catastrophic storm, extreme winds and flash flooding" across the northwest US Gulf Coast, the National Hurricane Center said.

The storm, located 320km south-southeast of Port Arthur in Texas, has maximum sustained winds of 230km/hr, the Miami-based forecaster said.

The hurricane has prompted warnings of an "unsurvivable" six metre storm surge and evacuation orders for hundreds of thousands of coastal residents of Louisiana and Texas.

It is set to make landfall along the coast of Texas and Louisiana overnight.

An "unsurvivable storm surge with large and destructive waves will cause catastrophic damage from Sea Rim State Park, Texas, to Intracoastal City, Louisiana," the NHC said.

Storm surges could penetrate up to 48km inland along parts of the coasts, and peak surge coupled with high tide could see water as high as 4.5 to 6 metres above normal levels.

Texas Governor Greg Abbott, speaking on the Weather Channel, urged residents to evacuate, saying "you have a few more hours to make sure you get out of harm's way."

Mr Abbott said: "This is a very dangerous storm, a stronger storm than what most have come through there.

"Do everything you can to get out of the way.

"Your property can be replaced. Your life cannot be replaced."

US President Donald Trump told residents in the path of the storm to "listen to local officials."

"Hurricane Laura is a very dangerous and rapidly intensifying hurricane," he tweeted. "My Administration remains fully engaged with state & local emergency managers."

Louisiana National Guard Sgt Aaron Dugas prepares a boat for the arrival of Hurricane Laura

Craig Brown, the acting mayor of Galveston, Texas, which suffered the deadliest hurricane in US history in 1900 with thousands of deaths, said the authorities were "monitoring this very closely."

Mr Brown said: "We've had good cooperation from our residents on evacuation" adding that it was not mandatory.

"If they want to stay put, then we allow them to do that," he said. "But we do tell them if they stay, they may not have any emergency services available to them."

Angela Jouett, director of evacuation operations in Lake Charles, said the authorities had new protocols in place because of the Covid-19 pandemic.

She said: "People that come in they get their hands sprayed with sanitiser. They're having their temperature checks, and we're also spacing everybody in six-foot distancing."

In New Orleans, devastated by Hurricane Katrina in 2005, the historic French Quarter was empty of tourists, while sandbags were piled up in front of the doorways of colonial-style buildings and windows were boarded up with plywood.

New Orleans remains traumatised from Katrina, which made landfall as a Category 3 storm, flooding 80% of the city and killing more than 1,800 people.

People purchase supplies at the Stine hardware store before the possible arrival of Hurricane Laura

Laura earlier caused flooding in Haiti and the Dominican Republic, killing at least 25 people.

In Cuba, it caused material damage but no deaths.

The Atlantic storm season, which runs through November, could be one of the busiest ever this year, with the NHC predicting as many as 25 named storms. Laura is the 12th so far.

Tropical Storm Marco, which also churned through the Gulf of Mexico, was downgraded from a hurricane and dissipated on Tuesday off the coast of Louisiana before reaching land.