Damage could be "catastrophic", the state governor said. Evacuation orders have been issued along the coast.
Matthew, with winds of 125mph (205km/h), is expected to strengthen and hit Florida as a Category Four storm, US officials say.
It has swept across the Caribbean, with the worst of the damage in Haiti.
At least 22 people have been killed in Haiti and thousands displaced. The storm has forced the presidential election there to be postponed.
Four people were also killed in the neighbouring Dominican Republic.