Florida has declared a state of emergency as Tropical Storm Erika, which has already wreaked havoc across the Caribbean, moves towards the US state.

The storm is expected to hit southern Florida on Monday after battering the Dominican Republic and Haiti with "life-threatening floods" over the weekend, the US National Hurricane Center said on Friday.

Governor Rick Scott made his declaration that Erika "poses a severe threat to the entire state" shortly after forecasters adjusted the trajectory of the storm to show that it is predicted to go through the middle of the state.

The order calls for the activation of the US National Guard and gives authorities the ability to waive tolls and rules to allow emergency crews and vehicles to move throughout the state.

A hurricane has not hit Florida in 10 years. The latest forecasts show that Erika will remain a tropical storm when it makes landfall.

Carribean havoc

On Friday, Erika lashed Puerto Rico with powerful winds and heavy rain a day after killing at least four people in the Caribbean island of Dominica.

Emergency officials were searching for several missing people after rain-triggered landslides on the small, mountainous island of on Thursday, Dominica's Prime Minister Roosevelt Skerrit said in a radio broadcast.

Overflowing rivers and landslides washed away several roads and bridges there, and Tourism Minister Robert Tonge posted photographs and video on Facebook showing widespread flooding in the capital.

The hurricane also battered the Virgin Islands as it moved across the Caribbean and appeared to head for the Dominican Republican, northern Haiti and eventually south Florida, the Miami-based National Hurricane Centre said.

Due to some likely weakening over the Dominican Republic, Erika was no longer forecast to make US landfall as a hurricane.

It could still hit the Miami area with sustained winds of 97km per hour on Monday, however, before sweeping northward up the Florida peninsula, affecting Orlando's popular theme parks.

The greatest risk was heavy rainfall over northern Haiti and the Dominican Republic, with up to 12cm in some areas. This could cause "life-threatening flash floods and mud slides," the hurricane centre said.

As Erika neared the Dominican Republic's capital, Santo Domingo, its sustained winds were measured at 80 kph, the centre said.

Source: Agencies