TAMPA: Tropical Storm Isaac lashed southern Florida Sunday and was expected to become a hurricane after forcing a one-day delay to the main events of the US Republican convention.

With winds reaching 65 miles (100 kilometers) per hour, the storm brought rain and choppy seas to the Florida Keys after battering Haiti and sweeping across Cuba late Saturday, the Miami-based US National Hurricane Center said.

A hurricane warning was in effect for the Florida Keys and parts of the state's southwest coast, and the Republican Party announced that severe weather warnings had postponed the start of its four-day gathering in Tampa.

According to the adjusted plan, the gathering will nominally open Monday and then immediately adjourn to reconvene on Tuesday, when the weather is expected to clear up.

Early Sunday, the eye of the storm was around 80 miles southeast of Key West, Florida and it was moving northwest at 18 miles per hour, with forecasts suggesting it would strengthen over the next 48 hours, the NHC said.

In its 11:00 am (1500 GMT) advisory, the NHC said Isaac was expected to be at or near hurricane strength as it passes over the Keys and predicted “hurricane conditions” there and in southwest Florida.

After sweeping across the string of islands off Florida's southern coast, the storm is on track to churn through the Gulf and make landfall Wednesday in Louisiana or Mississippi.

Tens of thousands of Republicans will be in Tampa for speeches, parties and the formal nomination of former Massachusetts governor Mitt Romney as the candidate to take on President Barack Obama in the November 6 election.

Republican National Committee Chairman Reince Priebus told CNN the decision was motivated by concern for delegates traveling in buses over long bridges and the difficulty of erecting tents to shelter security lines at venue entrances.

He insisted, however, that the show would go on, saying: “We're 100 per cent full steam ahead on Tuesday.”

Thousands of convention delegates are to stay in hotels along a coastal barrier island, which is likely to bear the brunt of the storm, and a decision was to be made on Sunday over whether to move them to higher ground.

Tampa Mayor Bob Buckhorn, a Democrat, had earlier sought to calm jitters about the weather, saying the storm might bring heavy rain to Tampa on the first of the four-day convention but would be followed by sunny skies.

“There may be wet shoes, but every day after Monday ought to be fine,” Buckhorn told Fox News on Saturday.

Vice President Joe Biden canceled a trip to Tampa and other Florida cities because of the approaching storm, Obama's Democratic campaign said.

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

Isaac was near hurricane strength early Saturday 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.

An eight-year-old Haitian girl died when a wall collapsed at her home and a 51-year-old woman died when her roof collapsed, according to officials, who later said two other people had also died in the storm.

More than 3,300 families had been evacuated to temporary shelters ahead of Isaac as aid groups provided clean water and hygiene kits to try to limit the risk of contaminated water and the spread of disease.

Cuba declared a state of alert in the island's six eastern provinces, where nearly five million people live, and evacuated around 5,000 foreign and local tourists from beachside hotels.