Tropical Storm Isaac gained power overnight as it roared toward Haiti with rains and high winds that could wreak havoc for people still living in makeshift shelters after a devastating 2010 earthquake.
Rains had already begun lashing southern Haiti, and US forecasters said the storm could be near hurricane strength when it smacks the southern coast of Hispaniola, the island Haiti shares with the Dominican Republic.
"Some strengthening is possible before the centre reaches Hispaniola tonight and Isaac could be near hurricane strength at landfall," the Miami, Florida-based National Hurricane Centre said in its 1800 GMT bulletin.
Some 400,000 people still live in temporary tent camps following the earthquake that killed some 250,000 people and devastated Port-au-Prince.
Long lines formed outside supermarkets in the Haitian capital as people filled stores to stock up on supplies.
Those still without proper shelter after the quake "remain amongst the most vulnerable, should the storm hit the city," said Jean-Claude Mukadi, Haiti's national director for the humanitarian group World Vision.
"Without a stable sanitation system or permanent housing, heavy rain and wind can create much larger problems like disease from water contamination."
President Michel Martelly, who cancelled a trip to Japan, earlier took to the airwaves with safety advice and to urge Haitians to follow the directions of civil defence personnel.
Prime Minister Laurent Lamothe said that the whole government, including security forces, has mobilised to prepare for the storm.
"We are going to work with our international partners to coordinate response actions," Lamothe said.
A hurricane watch was in effect for Haiti, and a tropical storm watch was in effect for the neighboring Dominican Republic and eastern Cuba.
The Haiti director for Oxfam, another humanitarian group, said that his group was preparing clean water and hygiene kits to help prevent the spread of cholera and other water-borne diseases.
"Nothing short of a miracle can keep people safe from this kind of storm when their only shelter is a tent," said Oxfam's Andrew Pugh.
"Haiti's disaster preparedness and response capacities have improved since the earthquake, but much remains to be done to help the poorest people cope with hurricane-strength threats," he said.
At 1800 GMT, Isaac was gathering strength and speed some 215 kilometres southeast of Port-au-Prince, the NHC said.
Its maximum sustained winds were clocked at 95 kilometers per hour as it moved in a westerly direction at 22 kilometres per hour, it said.
The storm could dump up to 51 centimetres of rain on Hispaniola.
"These rains could cause life-threatening flash floods and mudslides," the NHC said.
Schools were closed and hospitals added staff in at-risk areas of the Dominican Republic. Authorities urged people to prepare for floods, rising waters and mudslides due to heavy rains.
Isaac's centre was expected to hit the southern coast of Hispaniola on Friday afternoon local time, pass near or over Haiti overnight, and move near or over southeastern Cuba, home to the US naval base and "war on terror" prison at Guantanamo Bay, on Saturday and Sunday.
In Cuba, the government declared a state of alert in the island's six eastern provinces, where nearly five million people live.
Local authorities "must understand the possible impact of the intense rain on dams, canals and rivers," the Cuban civil defence office said, warning of blocked water drainage systems and flooded roads.
Cuba's civil defence system is designed to resist a US military invasion but has been effective at evacuating people and saving lives during past hurricanes.
Isaac could reach Florida as a hurricane early next week, just in time for the Republican Party's National Convention.
Tens of thousands of people from around the country will descend on Tampa for four days for the formal nomination of former Massachusetts governor Mitt Romney to challenge President Barack Obama in the November 6 election.
City officials have urged residents to prepare for the worst, and Republican National Committee chairman Reince Priebus has expressed concern about the storm but insisted the show would go on.