Tropical Storm Isaac is gaining strength, as the powerful storm takes aim at the Florida Keys.

Isaac’s winds have been clocked up to 100 kilometres an hour, and forecasters say they could reach official hurricane strength of 120 kilometres by the time the storm’s eye reaches the keys Sunday night.

Areas around the Florida Keys are also expected to be drenched by 150-millimetres of rain, which, along with the storm surge, has some worried about flooding.

In Tampa’s Pasco County, officials have already declared a state of emergency with flooding a big concern.

“Planning for the evacuation of some folks away from the coast possibly, and out of the areas that are already flooded, possibly” said one official.

Weather warnings and watches are currently in effect from Cuba to the Florida panhandle.

Some in beach areas have been warned to keep a three day supply of food and water on hand, as a precaution.

Officials in Florida have warned that if the peak storm surge comes at high tide some low-lying areas could be swamped with flooding up to 2-metres deep.