George Bush, the US president, declared a state of emergency in the state of Louisiana, as fears grew that another hurricane could strike the city of New Orleans three years after it was devastated by Katrina.
Along the Gulf Coast, US officials were preparing for the possibility of major evacuations should Gustav stay on course.
The hurricane was expected to be accompanied by storm surges as high as nine metres, officials said.
"This storm has the potential for being a very dangerous storm...so we strongly encourage the residents to follow the instructions of the local emergency management officials and be prepared to evacuate," Bill Irwin, a programme director with the US Army Corps of Engineers, which manages flood-control projects, said.
Bush's move to declare the state of emergency empowers federal authorities to lead all disaster relief efforts "to save lives, protect property and public health and safety, or to lessen or avert the threat of a catastrophe" in Louisiana, the White House said.
Oil prices rose in the face of Gustav's threat to the 4,000 oil facilities in the Gulf of Mexico that produce a quarter of US oil.
|New Orleans residents marked the anniversary of Katrina [Reuters]
Energy companies evacuated offshore workers and shut production in preparation for the most serious storm since the devastating 2005 Atlantic hurricane season.
On Friday, residents marked the third anniversary of hurricane Katrina's devastating impact on the city, when almost 1000 people were killed .
Many now face the possibility of an evacuation order to escape the expected landfall of the storm.
About 150 residents gathered in a cemetery and a lone trumpeter played a dirge as pallbearers guided a single silver casket from a horse-drawn carriage.
"We look forward to a better day, even as we prepare for a threat to come," said Ray Nagin, the mayor of New Orleans.
Gustav slammed into Haiti as a hurricane on Tuesday and killed at least 59 people there, eight in the neighboring Dominican Republic and five in Jamaica.
The storm weakened before buffeting Jamaica with high winds and heavy rains.
Four people drowned and one man fell to his death when a strong gust blew him out of a tree where he was picking breadfruit, police said.