At least three people were killed as Hurricane Frances relentlessly pounded Florida, US with gusty winds and heavy rain, leaving more than five million people without power, tearing off roofs and windows and leveling trees in its path.

The hurricane had started losing steam by evening and was downgraded to a tropical storm. But meteorologists warned that it could regain strength as it passes over the warm waters of the Gulf of Mexico and hit the four Panhandle counties of Florida.

It deposited more than 30cm of rain along the state's 670-km east coast, flooding large areas and submerging roadways.

The state, still reeling from the after-effects of Hurricane Charley, which hit less than three weeks ago, was preparing to clean up and count its losses even as officials warned residents to expect major destruction. Media reports quoted analysts as estimating that damage could be as high as $40 billion.

Frances struck the state's east coast with sustained winds of about 170km/hour but by the time it neared the west coast, the velocity had come down to 112km/hour.

Reports said some 100,000 residents had taken refuge in more than 400 shelters.

Florida Governor Jeb Bush surveyed the affected areas on Sunday after his brother - US President George W Bush - declared a 'major disaster' in the five worst-hit counties.