US President Donald Trump has issued a series of warnings to prepare for Hurricane Florence, which is gathering power as it approaches the Carolinas.
Mr Trump tweeted that this was "one of the worst storms to hit the East Coast in many years".
Florence is at present a Category Four storm, with sustained winds of about 140mph (220km/h).
On its current track it is predicted to make landfall near Wilmington, North Carolina, late on Thursday.
A number of mandatory evacuations have been ordered in South Carolina, North Carolina and Virginia, affecting more than a million people.
South Carolina's Governor Henry McMaster has given the entire coastline until noon on Tuesday to leave.
The University of North Carolina at Wilmington has also ordered a mandatory evacuation of students.
In its latest advisory at 23:00 eastern time (0300 GMT). the National Hurricane Center said of Florence: "Some strengthening is expected during the next 36 hours, and Florence is expected to be an extremely dangerous major hurricane through Thursday."
It said hurricane-force winds would "extend outward up to 40 miles (65km) from the centre and tropical-storm-force winds [would] extend outward up to 150 miles (240km)".
The 23:00 report placed Florence about 465 miles south-east of Bermuda, moving west-north-west at about 13mph.
This would see it pass between Bermuda and the Bahamas on Tuesday and Wednesday and make landfall on the North Carolina coast at about 22:00 local time on Thursday.
President Trump has signed approvals for the declarations of emergency in the Carolinas and said he had spoken to the governors of the affected states.
The US Navy is sending 30 ships stationed in Virginia out to sea.
Residents of affected areas have been flocking to stores to stock up on essentials.
Some petrol stations were running dry as customers filled up.