Hurricane Frances, a lumbering giant of a storm that had menaced Florida for days, started its slow march across central areas of the state on Sunday, moving at less than 10 miles per hour.

Forecasters at the US National Hurricane Center in Miami predicted the storm would move into the Gulf of Mexico on Monday morning. They placed much of Florida's north-west Gulf coast under a hurricane alert, warning that Frances could intensify once it entered the Gulf of Mexico. An early estimate of insured damage from the storm the second to strike the state in three weeks was between $2bn and $10bn, according to Risk Management Solutions, which provides risk assessments for the insurance industry.