President Obama has declared a state of emergency in Florida, Georgia and South Carolina
Northern Florida, Georgia and South Carolina remain on high alert as Hurricane Matthew sweeps along the east coast of the US with winds of 175km/h.
More than 800 people have already been killed as Matthew leaves a devastating trail through Haiti, Jamaica, the Dominican Republic and the Bahamas.
Matthew is now moving north towards northern Florida, Georgia and South Carolina, where hundreds of kilometres of coastline is still under threat of torrential rain and dangerous storm surge.
Speaking at the White House, President Obama stressed the threat to the Jacksonville area, and said he wanted to emphasise that "this is still a really dangerous hurricane".
In a media briefing he warned: "We can always replace property. We cannot replace lives."
The eye of the storm hung just offshore as it moved up the coastline, sparing communities the full force of its winds.
However, one man was killed by a falling tree in Florida's Volusia County when he went outside during a lull in the storm.
Forecasters have warned that the hurricane could still change course and head inland.
Its path is predicted to take it near or over the coast of northeast Florida and Georgia on Friday night, and near or over the coast of South Carolina on Saturday.
President Obama recalled the lessons of Hurricane Sandy, which slammed into New Jersey and New York City in 2012, leaving 200 dead and causing tens of billions of dollars in damage.
"Initially people thought 'this doesn't look as bad as we thought', and then suddenly you get a massive storm surge and a lot of people were severely affected."
Mr Obama warned that the potential for storm surge, flooding, loss of life and severe property damage "continues to exist" and that people need to be prepared in the days to come.
On Thursday, two million people living in low-lying areas and on the barrier islands in Florida were told to "evacuate, evacuate, evacuate" by Governor Rick Scott, who warned: "This storm will kill you."
Describing the hurricane as a "monster", he added: "Matthew is likely to produce devastating impacts."
The National Weather Service in Jacksonville, Florida, said tide levels in Charleston in South Carolina and Fort Pulaski in Georgia could approach or even surpass those seen during major floods in 2015.
Matthew has weakened slightly from to a Category 2 storm, but the National Hurricane Center says it is still a threat.
Airlines have cancelled at least 4,500 flights, including many in and out of Orlando, where all three of its world-famous theme parks - Walt Disney World, Universal Studios and SeaWorld - have closed.
Airports in southern Florida have started to return to normal however, with American Airlines seeing its first arrival at Miami at 9.05am local time.
At the Kennedy Space Center in central Florida, NASA and private company SpaceX were taking precautions to protect capsules and rockets.
The last Category 3 storm or higher to directly hit the US was Hurricane Wilma in October 2005, which killed dozens of people, caused billions of dollars in damage and left thousands of residents without power for more than a week.
'Storm hunter' Mark Robinson from the US Weather Network is sharing footage of winds in the Melbourne area of the state on Twitter.
The number of people killed by Matthew in Haiti has risen to 842, as medics on the island prepare for a surge in cholera cases.
The nation is left with flooded roads, collapsed bridges and power outages in the wake of the hurricane.
"Devastation is everywhere," said Pilus Enor, mayor of a town called Camp Perrin on the peninsula's south shore.
"Every house has lost its roof. All the plantations have been destroyed," he said.
"This is the first time we see something like this."
Four people have also been killed in the Dominican Republic, one in Colombia and one in St Vincent and the Grenadines.
But it is Haiti, already the hemisphere's least-developed and most aid-dependent nation, that has suffered the most.
Most deaths are believed to have occurred in the southwest region, and officials fear the toll might rise even further as information from remote areas trickles in.
The southern city of Jeremie is said to have suffered "complete destruction", while a bridge over the La Digue river in Petit Goave has been destroyed.
In Les Cayes, home to a major port, a cathedral roof has been completely blown off, and banana and mango crops have been ravaged in fields.
Officials say at least 350,000 people will need some sort of assistance, in what will likely become the country's worst humanitarian crisis since a devastating earthquake in January 2010.
With crops destroyed, wells inundated by seawater and some water treatment facilities ruined, officials said that food and water were urgently needed.