Gustav made landfall on Tuesday as a Category 1 hurricane on Haiti's southwestern coast near Jacmel, and began to slowly move inland.
Coastal storm surges, battering waves and heavy rain were predicted along Gustav's path. The hurricane is expected to veer near eastern Cuba by Wednesday, according to the National Hurricane Center.
Although the storm was expected to weaken slightly as it crossed Haiti, it could gain strength when it hits water again. Gustav could become a Category 2 hurricane, packing winds between 96 and 110 mph, and may become a major hurricane of Category 3 status over the next few days, according to forecasters.
Hurricane warnings and watches have been posted for Cuba and Jamaica.
American Airlines' flights from Fort Lauderdale and Miami to Haiti were canceled on Tuesday.
Gustav is the seventh named storm and the third hurricane (Bertha and Dolly preceded Gustav) of the 2008 Atlantic season.
The region is just now entering the peak of storm activity. Forecasters at the NHC also are watching three other systems that have potential for forming tropical storms.
Crude oil rose more than $2 a barrel on forecasts showing that Hurricane Gustav may enter the Gulf of Mexico, home to more than a fifth of U.S. oil production.