US president Barack Obama has declared a state of emergency in Florida as Hurricane Matthew approaches with winds of 140mph, leaving more than 100 dead in its wake across the Caribbean.
The hurricane gained fury as it closed in, growing from a possibly devastating Category 3 storm to a potentially catastrophic Category 4 by late on Thursday morning.
It was expected to scrape nearly the entire length of Florida's Atlantic coast from Thursday evening.
From there, forecasters said, it could push its way just off the coast of Georgia and South Carolina before veering out to sea.
Around two million people in Florida, Georgia and South Carolina were told to head for safety, and interstate highways were turned into one-way routes to speed up the exodus.
Forecasters said Matthew's fiercest winds appeared unlikely to strike Miami or Fort Lauderdale, the most densely populated areas in Florida, with about 4.4 million residents.
Those cities were expected to get tropical storm-force winds of between 39mph and 73mph.
Instead, forecasters said the West Palm Beach and Cape Canaveral areas further north could get the brunt of the storm.
More than 1.3 million people live in Palm Beach County and about 568,000 in Brevard County, home to Cape Canaveral and Nasa's Kennedy Space Centre.
Matthew killed at least 114 people in the Caribbean as it roared through Haiti, Cuba and the Bahamas.
Officials said at least 108 of those deaths were in desperately poor Haiti, where many towns were cut off by the storm and the magnitude of the death and destruction was just beginning to come into focus two days later.
In the Bahamas, authorities reported many downed trees and power lines but no immediate deaths.
With hurricane-force winds extending outwards up to 60 miles from the storm's centre, Matthew could wreak havoc along the coast even if it were to stay just offshore.
Forecasters said it could dump up to 15 inches of rain in some spots and cause a storm surge of nine feet.
Patients were transferred from two Florida waterfront hospitals and a nursing home near Daytona Beach to safer locations.
In inland Orlando, Walt Disney World, Universal Studios and SeaWorld announced plans to close early.
The Fort Lauderdale airport closed late in the morning, and the Orlando airport planned to shut down as well.
Airlines cancelled more than 2,800 flights scheduled for Thursday and Friday, many of them in or out of Miami and Fort Lauderdale.