The U.S. territory of Puerto Rico has issued a hurricane warning, as Tropical Storm Irene approaches from the east, already dumping heavy rain on the Leeward Islands.
A hurricane watch has been declared for the U.S. Virgin Islands.
Forecasters say Irene, with sustained winds of up to 85 kilometers per hour, could become a hurricane by Monday. Earlier, the storm was 375 kilometers east of Ponce, Puerto Rico, moving at 32 kilometers an hour.
The Dominican Republic, which shares the island of Hispaniola with earthquake-ravaged Haiti, issued a hurricane warning for its southern coast. Haiti's tropical storm watch has been upgraded to a warning.
A "watch" means a storm is possible, while a "warning" indicates a storm is definitely expected.
Irene is the ninth named storm of this year's Atlantic hurricane season.
Separately, Tropical Storm Harvey has been downgraded to a tropical depression after pounding Guatemala and eastern Mexico with heavy rain, prompting concerns about mudslides in the mountainous areas.