More than 1.5 million people have been ordered to evacuate their homes as Hurricane Florence heads towards the US eastern seaboard.
The storm is expected to make landfall in North and South Carolina in the next few days, with winds of up to 220 kilometres an hour.
The Carolinas, Virginia, Maryland, and Washington DC have all declared states of emergency.
Some residents have already left, but others have decided to stay and have been barricading their homes and stocking food supplies.
The governor of Maryland has said the flooding from Florence will potentially be catastrophic and life-threatening. Electricity supplies are also expected to be threatened by the wind.
US President Donald Trump has been briefed by the Federal Emergency Management Agency.
TRT World's Nicole Johnston is on the coast in Wilmington, North Carolina, where people are preparing for the worst.
The National Weather Service said that more than 5.4 million people live in areas now under hurricane warnings or watches on the US East Coast.
The Miami-based National Hurricane Center (NHC) said that Florence remained a Category 4 hurricane as of 1500 GMT packing deadly winds of 220 kilometres per hour.
"This storm is not going to be a glancing blow," said Jeff Byard, associate administrator for response and recovery at the Federal Emergency Management Agency (FEMA).
TRT World's Nicole Johnstone reports from Wilmington, North Carolina.
A state of emergency has been declared in Maryland, North Carolina, South Carolina, Virginia and Washington amid concern over potential torrential rain and flooding.
Washington Mayor Muriel Bowser said the emergency was "effective immediately" and "ensures that we will have the resources we need to prepare."
The last time the US capital declared a state of emergency was in January 2016 when a winter storm dubbed "Snowzilla" blanketed the capital region in knee-deep snow.
TRT World's Jay Gray reports from Carolina Beach, North Carolina.
Airlines are beginning to cancel flights ahead of the hurricane.
Charleston International Airport in South Carolina tweeted that it expects runways to close by midnight Wednesday as it monitors Hurricane Florence.
Southwest Airlines' website showed that it had cancelled at least a half-dozen flights to and from Charleston on Tuesday. The airline didn't immediately comment.
North Carolina is also evacuating prisoners and staff from low- and medium-security prisons that lie in the path of Hurricane Florence over concerns they won't be able to withstand the storm.
There are two big fuel pipelines in the path of Hurricane Florence, but analysts think the storm is unlikely to disrupt the flow of gasoline or other products.
TRT World's Sally Ayhan is tracking the hurricane in Washington DC and has the latest.
"This storm is going to be a direct hit," Byard told a press conference in Washington, warning of the potential for massive damage, flooding, power outages and loss of life.
Urging residents to evacuate, Byard said Florence was "the strongest storm to target the Carolinas and this part of the country in decades."
The NHC said Florence is expected to begin "re-strengthening later today and continue a slow strengthening trend for the next day or so."
"This is one of the worst storms to hit the East Coast in many years," Trump warned on Twitter. "Please be prepared, be careful and be SAFE!"
The US Navy sent about 30 ships stationed at its major bases in Virginia out to sea.
The vessels would get underway from Naval Station Norfolk and Joint Expeditionary Base Little Creek to avoid potential damage from winds and tidal surges, said Colonel Rob Manning, a Pentagon spokesman.
At this height of the Atlantic hurricane season, Florence was being trailed on east-to-west paths by two other storms, Hurricane Helene and Tropical Storm Isaac, but neither packs the deadly punch of Florence.