Winds of up to 75 kilometres an hour are threatening to cause further devastation to the earthquake-hit island of Haiti.
Tropical storm Isaac, which forecasters say could become a full hurricane, is expected to rip through the island and the Dominican Republican on Friday before heading to Cuba and the US.
Hundreds of thousands of Haitians still remain in tent cities following the magnitude 7.0 quake two years ago.
Prime Minister Laurent Lamothe says authorities are working to relocate those living under canvas to more secure temporary shelter.
One forecaster estimates the storm could disrupt oil and gas production in the Gulf of Mexico as it heads towards Florida.
Isaac is also threatening to pound the US city of Tampa where the Republican Party convention is due to get underway on Monday. Mitt Romney is expected to be given final backing to challenge President Barack Obama for the White House in the Autumn.
Officials say they could postpone the event if Isaac turns into a full hurricane.