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.
The weather system is currently a category four storm carrying sustained winds of about 215km/h.
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."
The latest National Weather Service (NWS) update on Tuesday morning says Florence is expected to "continue a slow strengthening trend for the next day or so".
It could weaken on Thursday, according to current predictions, but officials cautioned Florence will most probably remain "an extremely dangerous major hurricane through landfall".
Evacuations ordered for more than a million people
The NWS predicts potential storm surges of up to 3.6m on the coasts along with dangerously large swells and rip currents.
US President Donald Trump signed declarations of emergency on Tuesday for both North Carolina and South Carolina, freeing up federal money and resources for storm response.
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 gave the entire coastline of his state until noon on Tuesday to leave.
The Brunswick Nuclear Plant in Southport, North Carolina, is just a few miles from where Florence is expected to make landfall.
The chief meteorologist for WCBD-TV in South Carolina, Rob Fowler, told the BBC that Florence was getting bigger, and those even 100 miles away would feel the impact.
He said Florence could rival the impact of Hurricane Hugo, which wreaked $7bn (£5.3bn) in damage and claimed 49 lives in 1989.
The office of Virginia Governor Ralph Northam predicted "catastrophic inland flooding, high winds and possible widespread power outages".
Rainfall could be up to 20in (50cm) in some areas. This could worsen if, as some meteorologists predict, Florence stalls after it makes landfall.
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.
John Johnson, a hardware store manager in Charleston, South Carolina, told AFP news agency there had been a "nonstop" rush on batteries, flashlights, plastic tarpaulins and sandbags.
Some petrol stations were running dry as customers filled up.
Curtis Oil, a fuel distributor in Chesterfield, South Carolina, said it had been "overwhelmed with requests by state agencies and everybody else".
On the opposite side of the country, officials in Hawaii are warning residents and tourists to brace for the arrival of Tropical Storm Olivia.
Forecasters say Olivia - with current wind speeds of 65 mph - will make landfall on Wednesday morning.
Schools and state parks have closed. Ten to 15in (25 to 38 cm) of rain is forecast for the Big Island.