Haiti braced for a cruel new battering Thursday as Tropical Storm Isaac swept across the Caribbean towards the shattered island, gathering strength and threatening to reach hurricane force.

More than 400,000 citizens of the western hemisphere's poorest country were dangerously exposed to Isaac's gathering fury in makeshift squatter camps, two years after an earthquake destroyed the Haitian capital Port-au-Prince.

Meanwhile, residents in the neighboring Dominican Republic and on nearby Puerto Rico rushed to erect defenses against the expected wind and rain, which were to sweep on to Cuba and the southern United States by the weekend.

The Miami-based US National Hurricane Center said: "Isaac could become a hurricane on Friday before it reaches Hispaniola ... Preparations to protect life and property should be rushed to completion."

At 1800 GMT, Isaac's center was about 220 kilometers (135 miles) due south of Puerto Rico. It was expected to swing towards the Dominican Republic.

"Tropical storm conditions are expected over portions of Puerto Rico and the US and British Virgin Islands this afternoon. Hurricane conditions are expected over the Dominican Republic by Friday," an NHC statement said.

The storm will reach Haiti late Friday or early Saturday, it said.

Isaac was gathering speed and power. Winds within the storm are already sustained at 65 kilometers per hour, and storm force speeds could be measured 220 kilometers out from its center.

When it makes landfall on Hispaniola, the island divided between Haiti and the Dominican Republic, Isaac is expected to dump between 12 and 20 inches (30 to 50 centimeters) of rain on hillsides already scoured by soil erosion.

"These rains could cause life-threatening flash floods and mud slides," the center said, in its latest warning bulletin.

In southwest Puerto Rico, locals stocked up on water, fuel and supplies ahead of the storm, and restaurants, hotels, and homes tied down outdoor chairs and tables. Heavy rain could be seen far out to sea.

Haiti is particularly vulnerable because thousands of people are still living rough after the 2010 earthquake killed an estimated 250,000 people.

The storm was on track to make landfall in Cuba on Saturday at Guantanamo, the site of the US naval base and prison that houses the alleged September 11 plotters and other detainees from the so-called "War on Terror."

Several hundred people were evacuated from the base on Thursday, and legal proceedings against the alleged masterminds of the 2001 attacks on New York and Washington were postponed as the base buckled down for the storm.

Isaac could then reach Florida on Monday and Tuesday, just in time for the US Republican party's National Convention in Florida.

Republican delegates from around the country will be in Tampa for four days to formally nominate former Massachusetts governor Mitt Romney to challenge President Barack Obama in the November 6 election.

Isaac approaches Florida as the state is marking the 20th anniversary of Hurricane Andrew, a maximum level category five storm that killed 26 Americans and left some $26 billion in material damage.

The state's governor Rick Scott, while saying it was still early to predict Isaac's path, urged residents to prepare for the worst, saying: "Every family should be prepared to sustain themselves for up to 72 hours."

Meanwhile, a second tropical depression gained strength in the eastern Atlantic and became Tropical Storm Joyce, though there was no immediate threat to land and it was moving relatively slowly without gaining power.

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