Oil companies began evacuating workers from rigs in the Gulf of Mexico on Thursday ahead of what US forecasters said could be a powerful and dangerous storm.

"Right now we're giving it a 70 percent chance of developing into a tropical cyclone sometime in the next 48 hours," said Dennis Feltgen, a spokesman for the Miami-based National Hurricane Center.

"Where it will go and what it will do is too early to tell."

The weather service is urging residents across the Gulf Coast to keep an eye on the storm which will likely bring heavy rain and powerful winds and could also cause some flooding.

"Gosh knows the rain is needed from Texas to Florida," Feltgen told AFP. "This could be a godsend."

But it could spell a slowdown for oil operations in the Gulf, source of a substantial portion of US offshore crude.

ExxonMobil said it is evacuating approximately 140 employees and contractors from Gulf Coast offshore platforms expected to be in the path of the storm.

"Gross production of approximately 11,000 barrels per day of liquids and 60 million cubic feet per day of natural gas has been shut-in," the oil giant said in a statement.

BP said it had begun evacuating "all personnel from its operative assets in the Gulf of Mexico" on Thursday, but did not indicate the impact on production. Shell said weather conditions were interfering with evacuation efforts and it might be safer to leave some workers on their rigs.

"Our priorities at this time continue to be ensuring the safety of personnel, protecting the environment and minimizing production and operational impact," the Dutch energy giant said in a statement.

Shell said it had begun to take actions to shut in some production, primarily in "subsea fields that require specific treatments to ensure the production can be restored after the storm passes," and that the impacts so far are "minimal."

Anadarko said it had shut in production at all eight of its facilities and evacuating all personnel -- about 100 people.

Chevron said it was evacuating "non-essential personnel" and that production has not been affected.