Hundreds of people may be trapped on a North Carolina island amid rising floodwater after Hurricane Dorian made landfall.
The storm struck the islands popular with holidaymakers days after causing at least 30 deaths in the Bahamas.
State governor Roy Cooper warned of storm surges on the island of Ocracoke, where 800 people were said to have stayed behind.
In the Bahamas, officials say hundreds, possibly thousands, are still missing. The official death toll is expected to increase to a "staggering" level, the country's government says.
Officials are sending morticians and 200 body bags to the Abaco Islands, the worst-hit part of the archipelago.
What is happening on Ocracoke?
The storm made landfall at Cape Hatteras at 9am local time (13:00 GMT), with winds of 150 km/h, but within two hours its eye had moved about 70km to the north-east.
However, it lashed the Outer Banks islands with hurricane force winds as far as 60km away from the eye and with tropical storm winds up to 300km away, the National Hurricane Center said.
Mr Cooper said there was "significant concern" for those reportedly trapped on Ocracoke and urged them to get to high ground. Rescue teams were ready to deploy as soon as the area became accessible, he said.
Local weather officials posted video of a storm surge causing severe flooding in Ocracoke.
We're getting some pictures and videos of the severe flooding due to storm surge on the Outer Banks. This is Ocracoke, NC. Our hearts go out to ALL impacted by #Dorian. #nws pic.twitter.com/OAVxjJkUjj— NWS Newport/Morehead (@NWSMoreheadCity) September 6, 2019
Officials in Hyde County were preparing to airlift residents to a shelter and said they would continue the evacuation by ferry as soon as possible.
Now downgraded to a category 1 hurricane, Dorian is moving north-east at about 27km/h. It is expected to make landfall again tomorrow in the Canadian province of Nova Scotia.
Life-threatening storm surges and dangerous winds are still expected in Virginia and other north-eastern states.
Four deaths so far in US
Three storm-related deaths have been reported in Florida and another in North Carolina. All four people died while preparing their homes and shelters for the storm.
It has also caused severe flooding and widespread power failures in Georgia, South Carolina and North Carolina.
In the Carolinas, more than 900,000 people were ordered to evacuate their homes, but it was unclear how many had complied.
Forecasters said 25cm of rain had fallen between the coasts of Charleston in South Carolina and Wilmington 270km away in North Carolina.
Bahamas island uninhabitable
Dorian hit the Bahamas as a category five hurricane with winds reaching 298km/h. It matched the highest ever recorded at landfall, and stayed over affected areas for two days.
In its wake, at least 70,000 Bahamians needed immediate humanitarian relief.
The International Red Cross fears 45 percent of homes on Grand Bahama and the Abacos - some 13,000 properties - were severely damaged or destroyed.
Parts of the Bahamas received up to 89cm of rain, leaving vast areas of it flooded.
The island of Great Abaco is virtually uninhabitable, with bodies piled up, no water, power or food, and militias formed to prevent looting, local media report.
Aerial images over the Abacos showed huge destruction, with roofs torn off, scattered debris, overturned cars, shipping containers and boats, and high water levels.
The only international airport in Grand Bahama was also devastated. As a result, it cannot be used for evacuations or aid deliveries.
Flood waters have reportedly rendered the Grand Bahamas' main hospital unusable, according to the Associated Press news agency.
Aid efforts ramping up
The Bahamas' government has signed an emergency declaration giving tax exemptions to medicine, building supplies and other goods that can be used for relief efforts.
International aid operations have been primarily focused on Grand Bahama and the Abacos, which were hit worst.
Search-and-rescue teams have been combing the worst-hit communities looking for any trace of survivors and bodies. As of Friday, the US Coast Guard said it had rescued 205 residents.
Meanwhile, efforts to deliver aid are being ramped up. Planes and helicopters have been flying in emergency supplies to help the estimated 76,000 people in need of food and shelter.
Eight tonnes of ready-to-eat meals, storage units, generators and other emergency suppliers are to be flown in from Panama, the UN World Food Programme said.
A $US5.4m ($NZ8.4m) budget has been allocated to the UN agency for a three-month emergency operation in the Bahamas.