Miami  - Hurricane Gustav had gathered strength early Tuesday in the Caribbean Sea and was threatening Haiti, the Dominican Republic, Cuba and Jamaica, forecasters said.

Gustav could produce 10 to 20 centimetres of rain over Hispaniola, eastern Cuba and Jamaica, with some areas receiving up to 50 centimetres, the US National Hurricane Centre in Miami said.

"These rains will likely produce life-threatening flash floods and mud slides," its latest report said.

The NHC said Gustav formed unexpectedly early Tuesday, south of the island of Hispaniola, where the countries of Haiti and the Dominican Republic are located.

It has maximum sustained winds of 150 kilometres per hour. The category 1 hurricane on the five-level Saffir-Simpson scale was expected to move over south-western Haiti later Tuesday and near eastern Cuba on Wednesday.

The hurricane had already caused heavy rain and strong winds in Haiti and the Dominican Republic by Tuesday. The authorities of both countries decreed maximum alert levels in the affected areas, where flooding, flash floods and mudslides were feared.

Some 4,360 people had to be evacuated and more than 870 homes suffered damage in the Dominican Republic. Emergency services in the country said no one had been reported dead in connection with the hurricane.

One flight from the Dominican Republic to Aruba had to be called off, and boats were asked not to leave port in either country on Hispaniola.

Last week, Tropical Storm Fay left 11 people confirmed dead and some 50 missing and caused substantial damage to agriculture in Haiti and the Dominican Republic.

Central Cuba was also on alert, and its Meteorology Institute (ISMET) warned that the storm was gaining strength.

Gustav is the seventh storm in the current north-Atlantic hurricane season, and the third hurricane. The season officially ends at the end of November. (dpa)