-- With the hours counting down until Hurricane Matthew is expected to slam into the United States' southern Atlantic coast, widespread evacuations are taking effect as residents brace for the most powerful Atlantic storm in almost a decade.
A hurricane warning is in effect for parts of Florida, from Golden Beach north to Sebastian Inlet, as well as Lake Okeechobee. The area north of Sebastian Inlet to Fernandina Beach is under a hurricane watch.
A tropical storm warning is in effect from Chokoloskee to Golden Beach, for the Florida Keys east of Seven Mile Bridge and for Florida Bay, according to the National Hurricane Center.
The governors of Florida, Georgia, North Carolina and South Carolina have already declared states of emergency. They issued stern warnings for millions of coastal dwellers to prepare to evacuate as the Hurricane Matthew moves north-northwest at about 12 mph.
He said this could be the largest evacuation that Florida has ever experienced.
Florida residents in coastal areas of Duval County — including Jacksonville Beach, Neptune Beach and Atlantic Beach — have been advised to evacuate as soon as possible today. Mandatory evacuations have been ordered for Brevard County's barrier islands and for Martin County's Hutchinson Island, Jupiter Island, Sewall's Point and mobile homes. In Palm Beach County, recommended but not mandatory evacuations are in place for the barrier islands from Boca Raton to Jupiter. Voluntary evacuations have begun in Broward, Flagler, Indian River and St. Lucie counties.
In Miami-Dade County, bridges will shut down at 8 p.m. Eastern time, before the storm hits. But Miami Mayor Carlos Gimenez said there are currently no evacuation plans.
Long lines at gas stations and empty shelves at grocery stores were reported in cities across the southeastern United States as residents stocked up on supplies and prepared to flee. Many schools are closed, and people have boarded up their beachside homes.
South Carolina residents in Charleston and Beauford counties and areas of Berkeley, Colleton, Dorchester and Jasper counties should begin evacuating today no later than 3 p.m. Eastern time. The scope of the evacuation may expand Thursday to include parts of Horry and Georgetown, Haley said.
"The storm did slow down, and it did move somewhat, but we are not in stable territory yet," she told reporters at a news conference this morning. "If you do not leave, you are putting a law enforcement officer or national guardsman's life on the line when they have to go back and get you."
In North Carolina, Cape Lookout National Seashore in Carteret County is being evacuated before the storm. Superintendent Patrick Kenney said more than 100 trucks were being removed from the barrier islands and about 45 cabins were being cleared.
And in Georgia, the Glynn County Board of Commissioners has called for a voluntary evacuation of threatened islands: St. Simons, Jekyll, Sea and Little St. Simons. Officials said mainland evacuation is not anticipated at this time.
The U.S. Coast Guard is warning the public to stay off the water, secure boats, stay clear of beaches and follow evacuation orders because its search and rescue capabilities will be limited as Hurricane Matthew strengthens.
After a briefing at FEMA headquarters in Washington, D.C., today, President Barack Obama told reporters Hurricane Matthew is "a serious storm, and we want everybody to take it seriously as well."
"Just remember that you can always rebuild," he said. "You can always repair property. You cannot restore a life if it is lost, and we want to make sure that we minimize any possible loss of life or risk to people in these areas."
All U.S. airports remain open and fully operational. So far, flight cancellations are limited to flights to, from or through the Caribbean. The major airlines are offering customers waivers of change fees for flights to, from or through affected areas in the southeastern United States.
ABC News' Jeffrey Cook, Erin Dooley, Max Golembo, Melissa Griffin, Alex Mallin, Ben Stein and Jason Volack contributed to this story, which was supplemented with Associated Press reporting.