CHICAGO (AFP) - A major storm, Harvey, was upgraded to hurricane status on Thursday (Aug 24), as it targeted hundreds of kilometres of coastline in Texas and Louisiana.

The US National Hurricane Centre (NHC) warned of a potential for "life threatening" floods from the storm, which was due to make landfall sometime on Friday.

The NHC said the category one hurricane could hit land as a much more powerful category three, with winds of 209kmh.

Harvey also was expected to deluge flood-prone New Orleans in neighbouring Louisiana, where the catastrophic Hurricane Katrina in 2005 caused widespread flooding and killed more than 1,800 people.

"We could see some localised flooding," New Orleans Mayor Mitch Landrieu told a news conference on Thursday, adding that sandbags, boats and high-water rescue vehicles were being readied, although no evacuations were immediately planned.

"We just need to make sure that we're prepared for heavy rain over the course of the next week," Landrieu said.

Harvey's wind speed on Thursday was about 129kmh.

A storm is categorised as a category one hurricane when it has winds of 119kmh.

The slow-moving storm was expected to dump as much as 75cm of rain in some parts of Texas. Dangerous storm surges were also forecast, reaching between six to 10 feet above ground level in the worst-hit regions.

Even though landfall was still hours away, Governor Greg Abbott on Thursday issued disaster declarations in 30 counties, saying the preemptive move would allow "Texas to quickly deploy resources for the emergency response effort."

Officials in Houston, the biggest city in the path of the storm, said they did not anticipate issuing evacuation orders, but expected heavy rainfall to last up to five days.