A strengthening Hurricane Gustav swept over the Cayman Islands as it headed toward Cuba and the Gulf of Mexico on the third anniversary of Hurricane Katrina's deadly strike on New Orleans.
The storm, which has killed up to 77 people in the Caribbean, was plowing toward superheated waters near Cuba where it could absorb enough energy to strengthen into a major hurricane.
It is forecast to rip through the heavy concentration of US oil and natural gas platforms off Louisiana.
The US National Hurricane Centre says Gustav could be a "major" Category 3 storm.
Oil prices slipped after a week of volatile trading due to Gustav's threat to the 4,000 Gulf platforms that produce a quarter of US oil.
New Orleans has begun voluntary evacuations. Authorities are considering ordering mandatory evacuations to prevent a repeat of the devastation and deaths of Hurricane Katrina.
Louisiana and Mississippi have already been declared states of emergency.
Katrina killed 1800 people on the US Gulf Coast, most of them in New Orleans.