Tropical storm Gustav, which has killed dozens of people in the Caribbean, has now strengthened to hurricane status. It is expected to build to a Category 3 storm by the time it hits the United States next week.
The storm killed at least 72 people in Dominican Republic, Haiti and Jamaica, and is now heading towards the Cayman Islands.
Meteorologists say it could become a major hurricane before reaching the Gulf of Mexico.
About a quarter of US oil output and 15% of natural gas production is in its path.
The price of US crude oil rose $US3 early on Friday to more than $US118 per barrel because of possible disruption to supply.
New Orleans has begun voluntary evacuations ahead of the Gustav's projected arrival next week on the third anniversary of Hurricane Katrina.
Authorities are considering ordering mandatory evacuations to prevent a repeat of the devastation and deaths of 2005.
Louisiana and Mississippi have already declared states of emergency.
Katrina killed 1,800 people on the US Gulf Coast, most of them in New Orleans.