Tropical storm Fay threatened to strengthen into a hurricane as it moved towards Cuba after dousing Haiti and the Dominican Republic yesterday with torrential rains and killing at least one person.

The sixth cyclone of what experts predict will be an unusually busy Atlantic hurricane season, Fay was expected to be near hurricane strength when it approaches central Cuba today, and at hurricane strength over the Florida Keys and off Florida's west coast after that, US forecasters said.

Areas of the Gulf of Mexico where around a quarter of US oil and 15% of US natural gas are produced did not appear to be at immediate risk.

But long-range storm forecasts are prone to error, especially when it comes to intensities, and Shell Oil Co said it was pulling 200 workers from offshore operations in the eastern Gulf of Mexico ahead of the storm.

The more time that Fay spends over the warm waters that provide tropical storms with fuel, the stronger it will get.

The government of Cuba issued a hurricane watch, signalling hurricane conditions could be expected within 36 hours, for the central provinces of Camaguey, Ciego de Avila and Sancti Spiritus, the US National Hurricane Centre said.

A 34-year-old Dominican woman died, and her two nephews, aged 13 and five, were missing after being swept away when flood waters raged through a gully around 86 miles east of Santa Domingo and engulfed their truck, the Dominican Republic's emergency operations centre said.

By 6pm GMT, Fay's centre was located around 50 miles south- southeast of Guantanamo, Cuba, and it was moving west at 16 miles per hour, the Miami-based hurricane centre said.

Despite the mountainous terrain of Haiti, which had been expected to hamper the storm's development, Fay held together quite well overnight after forming on Friday as it moved ashore in the Dominican Republic.

The storm's forecast track would take it over central Cuba, past Key West and then into Florida's Gulf Coast as a hurricane, the centre said. A tropical storm becomes a hurricane when its top sustained winds reach 74 mph.

Hurricane Katrina was a monstrous Category Five hurricane in the Gulf of Mexico in August 2005 before coming ashore as a Category Three storm and flooding New Orleans in the most costly natural disaster in US history.

The immediate threat from Fay was the heavy rainfall of four to eight inches that it was bringing to Hispaniola, the island shared by Haiti and the Dominican Republic. In some areas, torrential rainfall of 15 inches could be expected.

Haiti in particular is vulnerable to mudslides and floods because its hillsides have been stripped of trees by people seeking charcoal for cooking. Around 3000 Haitians died in September 2004 when the then tropical storm Jeanne sent a river of mud into the port city of Gonaives.

In addition to the hurricane alert in Cuba, tropical storm warnings and watches were in effect for Haiti, the Turks and Caicos, the southeastern Bahamas, the Cayman Islands and Jamaica.