CHICAGO • Millions of people in the US Gulf Coast states of Texas and Louisiana braced themselves for the arrival of Hurricane Harvey as it intensified to Category 2, with winds whipping up to 165 kmh.

The United States National Hurricane Center (NHC) warned that Harvey was "dangerously approaching" the Texas coast and creating a potential for "life-threatening and devastating" floods as it roared towards an area that processes some seven million barrels of oil a day.

The storm's centre was due to make landfall in Texas late last night or early today local time (this morning Singapore time).

Harvey is forecast to come ashore as a Category 3 hurricane - the third-most powerful on the Saffir-Simpson scale - NHC said. That would make it the first major hurricane to hit mainland US since Hurricane Wilma struck Florida in 2005.

"For anyone who has not already evacuated, please hurry to do so," the city of Portland, Texas, declared on its website in capital letters.

The storm was threatening one-third of the US refining capacity, forcing several energy companies to take precautions and evacuate personnel from oil and gas platforms in the heart of the US "Refinery Row".

One oil-drilling rig in the Gulf of Mexico was evacuated on Thursday, as well as 39 manned oil and natural gas production platforms, according to the Bureau of Safety and Environmental Enforcement.

Those evacuations represented an estimated 9.5 per cent of oil production and 14.7 per cent of natural gas production in the Gulf, the bureau said.

NHC expects Harvey to move slowly over Texas and linger over the state for days.

Houston, the nation's fourth-most populous city, warned residents of flooding from close to 50cm of rain over several days.

Up to 97cm of rain is expected over parts of Texas, with winds of up to 200kmh, and sea levels may surge as high as 3.7m. Louisiana could get 25cm to 38cm of rain. Flood warnings are in effect for Louisiana and northern Mexico.

Louisiana and Texas have declared states of disaster, authorising the use of state resources to prepare for the storm. President Donald Trump has been briefed and is ready to provide resources if needed, the White House said on Thursday.

In New Orleans, Louisiana, where Hurricane Katrina in 2005 caused widespread flooding and killed more than 1,800 people, Mayor Mitch Landrieu told journalists that high-water rescue vehicles and boats were ready to be deployed.

"We just need to make sure that we're prepared for heavy rain over the course of the next week," Mr Landrieu said, adding that there were no evacuations planned.

This is the first major storm of the annual Atlantic hurricane season.

"The tropical Atlantic remains warmer than normal," said Dr Phil Klotzbach, an atmospheric scientist at Colorado State University.

"So we are expecting an above-average overall season."

AGENCE FRANCE-PRESSE, REUTERS