Hurricane Florence - the most powerful storm to threaten the Carolinas in nearly three decades - is expected to strengthen, say forecasters.
Officials warn there is a chance of "life-threatening inundation from rising water" over the next 48 hours as Florence heads for the US East Coast.
With sustained winds of about 140mph (225km/h), the weather system is currently a category four storm.
It could make landfall on Thursday, near Wilmington, North Carolina.
Virginia, Maryland, Washington DC, and North and South Carolina have all declared states of emergency.
"This storm is a monster," North Carolina Governor Roy Cooper told a news conference on Tuesday. "It's big and it's vicious.
"It is an extremely dangerous, life-threatening, historic hurricane."
Where is Florence now?
Hurricane Florence was 670 miles (1,078km) south-east of Cape Fear, in North Carolina, as of 23:00 Atlantic Standard Time on Tuesday (03:00 GMT Wednesday), according to the latest National Hurricane Center update.
The update says Florence - which is moving north-west at about 17mph (28km/h) - is expected to strengthen on Tuesday night and on Wednesday.
Florence is expected to slow down considerably by late Thursday into Friday.
What damage could it do?
The National Weather Service predicts potential storm surges of up to 13ft (4m) on the coasts along with dangerously large swells and rip currents.
Hurricane-force winds will extend outward up to 60 miles from the centre and tropical-storm-force winds up to 175 miles.
What are people doing to prepare?
Mandatory evacuations have been ordered in parts of South Carolina, North Carolina and Virginia, affecting more than a million people.
South Carolina's Governor Henry McMaster initially ordered the entire coastline of his state to leave. However, the order was later confined to the northern coast.
Residents of affected areas have been flocking to stores to stock up on essentials.
Why are people worried about flood insurance?
The US government has a National Flood Insurance Programme (NFIP) that helps pay for flood coverage in high-risk zones across the country.
The programme, which is billions of dollars in debt, has been up for renewal several times - Congress' next deadline to re-authorise the NFIP is 30 November.
While some wind damage may be covered by regular homeowners' insurance, flooding is a different story.
In the US, flood protection policies from private providers are often very expensive, largely because it is difficult to assess a home's flood risk.
These policies also take 30 days to kick in, meaning any homeowners without coverage in the Carolinas now will not be able to insure their property before Florence hits.
According to the Insurance Information Institute, 14% of Americans living in the southern states had flood insurance as of 2016.