Tropical Storm Isaac battered impoverished Haiti early Saturday as a hurricane warning was issued for southern Florida days before the opening of the US Republican convention.

With wind gusts of up to 60 miles (95 kilometers) per hour, the storm was expected to sweep over eastern Cuba later Saturday and to become a hurricane on Sunday as it nears Florida, according to the US National Hurricane Center.

A hurricane warning was in effect for the Florida Keys and parts of the southwest coast, the Miami-based NHC said, raising concerns the storm could disrupt the Republican National Convention opening in Tampa on Monday.

The forecasters said Isaac was near hurricane strength when the eye of the storm passed over Haiti, where hundreds of thousands of people are still living in squalid, makeshift camps following a catastrophic 2010 earthquake.

Haiti was the poorest country in the Western Hemisphere even before the earthquake killed 250,000 people, and some 400,000 are still living in tent camps in and around the devastated capital Port-au-Prince.

The NHC warned of "life-threatening flash floods and mud slides" on the island of Hispaniola -- shared by Haiti and the Dominican Republic -- and tropical storm conditions in eastern Cuba and the Bahamas.

The streets of Port-au-Prince were empty late Friday, with only a few vehicles venturing out after dark. Earlier, long lines had formed outside supermarkets as people stocked up on supplies.

"We're not ready," said Martine, who heads a watch group at a camp hosting a thousand families.

"When it rains, we stand under tents with holes in them. There are many children and we don't know what to do if we have to evacuate," she told AFP.

With no access to public restrooms or safe drinking water, residents of the Canape Vert camp complained about the lack of help from the authorities.

But a government official said President Michel Martelly, who canceled a trip to Japan, had toured emergency shelters in central Port-au-Prince to distribute food supplies and blankets.

Prime Minister Laurent Lamothe said the entire government, including security forces, had mobilized to prepare for the storm.

Aid groups warned that those without proper shelter after the quake were among the most vulnerable if Isaac hits the capital, at risk of disease from water contamination and other disaster scenarios.

Oxfam said it 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 the group's Haiti director, 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."

Isaac was 95 miles (150 kilometers) southeast of Cuba as it swirled to the northwest at 14 miles (22 kilometers) per hour, the NHC said in its latest bulletin, issued at 8:00 am (1200 GMT).

After striking Haiti and southeastern Cuba, home to the US naval base and "war on terror" prison at Guantanamo Bay, Isaac was due to head Sunday for the Florida Keys off the southern tip of the United States.

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 defense office said, warning of blocked water drainage systems and flooded roads.

Isaac could reach Florida early Monday just in time for the Republican convention, when tens of thousands of people will descend on Tampa for speeches, parties and the formal nomination of former Massachusetts governor Mitt Romney to take on 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.

Vice President Joe Biden has meanwhile canceled a trip to Tampa because of the approaching storm, the Democratic campaign said.

In the Gulf of Mexico, BP evacuated its Thunder Horse platform, the world's largest offshore production and drilling facility.