CORPUS CHRISTI, United States (AFP) — Harvey strengthened yesterday into a dangerous category three hurricane as it barrelled toward the US Gulf Coast, where residents of Texas and Louisiana fled the monster storm, expected to be the most powerful to hit the US mainland since 2005.

Texas Governor Greg Abbott asked President Donald Trump to pre-emptively declare Harvey a “major disaster” in order to speed federal assistance, issuing disaster declarations for 30 counties.

The arrival of the storm was likely to be a major test for the Republican president, who the White House said would head to the affected region early next week.

“We can obviously tell already at this stage this is going to be a very major disaster,” a sombre Abbott said, as more than 1,000 members of the National Guard were activated.

“We’re going to be dealing with really record-setting flooding in multiple regions.”

Highways leading from coastal areas were jammed as authorities issued urgent warnings to hundreds of thousands of residents to flee. Some highways were to be turned into one-way roads to speed the exodus from the storm zone.

Harvey, the first major storm of the annual Atlantic hurricane season, was packing maximum sustained winds of 120 miles (195 kilometres) per hour.

As of 1900 GMT, it was located about 75 miles east-southeast of Corpus Christi, a major hub for the American oil industry, and moving at 10 mph. Coastal water levels were already rising, it said.

The storm is expected to make landfall in the populous coastal part of Texas by early Saturday, dumping up to 35 inches (89 centimetres) of rain over a four- or five-day period.

“All the advice we can give is get out, and get out now,” said Patrick Rios, the mayor of Rockport, Texas, where an estimated 60 per cent of the town’s 9,500 residents had left.

Rios had blunt words for those determined to stay, telling them to “mark their arm with a Sharpie pen, put their social security number” — to be identified if found dead.

As he headed to the Camp David presidential retreat for the weekend with his family, the president said: “Good luck to everybody.”

Satellite images showed a mammoth storm system extending hundreds of miles into the Gulf.

It is forecast to be the most powerful hurricane to hit the mainland since Wilma struck Florida in 2005, and could inflict billions of dollars in damage.

The year 2005 was huge for hurricanes — before Wilma, Hurricane Katrina pummelled New Orleans, leaving more than 1,800 dead and becoming one of the greatest hiccups in the presidency of George W Bush.

Coastal Texas is a fast-growing area, with some 1.5 million people moving into the area since 1999. Authorities said the combination of dense growth and perhaps a year’s worth of rain falling in just days could prove deadly.

Local television footage showed supermarket aisles plucked bare, houses and shops with windows boarded over, and long lines snaking outside gas stations.

The National Hurricane Center warned of the “complete destruction of mobile homes”, of many buildings “washing away”, and some areas being left “uninhabitable for weeks or months”.

Before the storm hit, the US Federal Emergency Management Agency (FEMA), prepositioned emergency supplies, and state authorities mobilized some 1,000 army and air national guard troops to help with evacuation and recovery.

In 2005, Bush faced severe criticism after FEMA appeared unprepared for the devastating damage inflicted by Hurricane Katrina.

“Keep on top of hurricane Harvey, don’t make same mistake Pres Bush made w Katrina,” Republican Senator Chuck Grassley urged the US leader in a tweet.

The president, in several tweets, said he was closely monitoring the storm’s progress and said he was “here to assist as needed”.

The White House said Trump was being briefed regularly on the storm, had spoken to the Texas and Louisiana governors, and planned to visit Texas early next week.

“I encourage everyone in the path of #HurricaneHarvey to heed the advice & orders of their local and state officials,” Trump tweeted.

“This storm will likely be very destructive for several days,” the White House said.

In Corpus Christi, a children’s hospital evacuated 10 newborn infants to a facility inland due to the prospect of extended power outages.

But many Corpus Christi residents appeared bent on sitting the storm out, packing sandbags to protect their homes — until the supply of sandbags ran out.

Sheriff Frank Osborne of Matagorda County, where evacuations were mandatory, described the high stakes to TV station KHOU.

“If you’re in a mandatory evacuation area and you don’t evacuate, don’t call us. Because I’m telling you right now, I’m not going to put one of my deputies’ life on the line to save somebody that didn’t leave when they were asked to,” he said.

People east of the mammoth, three-mile (five-kilometre) seawall in Galveston Island were urged to evacuate. Galveston was the scene of the devastating 1900 hurricane that claimed an estimated 8,000 lives.

Officials in Houston, the biggest city in the storm’s path, cancelled school but did not anticipate an evacuation.

Inland cities like San Antonio prepared to welcome evacuees, and some began arriving on Friday.

FEMA chief Brock Long said the most pressing danger was the storm surge, the high tides powered by powerful winds — expected to reach between six and 12 feet in some areas — but said many inland counties should prepare for “significant” flooding.

Meteorologist Eric Holthaus told AFP the prospect of the storm stalling on the coast, lashing it with heavy rain for days, “is just terrifying”.

“This is the sort of storm that meteorologists prepare for and think about for years,” he said.

One-third of the US refining capacity is potentially under threat.

One oil drilling rig in the Gulf of Mexico has been evacuated, as well as 39 manned production platforms, according to the Bureau of Safety and Environmental Enforcement, representing an estimated 9.5 per cent of oil output and 14.7 per cent of natural gas output in the Gulf.

In Louisiana, Governor John Bel Edwards described the situation issued an emergency declaration for his entire state.

Authorities in New Orleans readied high-water rescue vehicles and boats.

Edwards described a “worse-case scenario” in which the storm leaves Texas, gains new strength over the heated waters of the Gulf, and then heads toward Louisiana.

“All of that’s possible,” he said. “And the best forecasters, the best meteorologists that we have available… they can’t tell us yet.”