A mother and infant were killed in North Carolina when a tree fell on their house, as Hurricane Florence crashed into the state, swamping streets with torrential rain and a powerful storm surge.
Police say the pair, in Wilmington, were the first confirmed deaths directly related to the storm, which made landfall earlier on Friday.
The child's father was taken to hospital.
In Pender County, North Carolina, one woman died after she suffered a heart attack and paramedics were unable to reach her due to blocked roads, authorities said.
After making landfall, Florence slowed to a pace that meant it would plague the area with days of flooding.
The wall of water pushed in from the Atlantic 'overwhelmed' New Bern, a town of about 30,000 people at the confluence of the Neuse and Trent rivers, North Carolina Governor Roy Cooper said.
'The sun rose this morning on an extremely dangerous situation and it's going to get worse,' Cooper told a news conference in the state capital Raleigh.
'To those in the storm's path, if you can hear me, please stay sheltered in place.'
Cooper said Florence would 'continue its violent grind across the state for days.'
More than 60 people, including many children and pets, had to be evacuated from a hotel in Jacksonville, North Carolina, after strong winds caused parts of the roof to collapse.
The centre of the hurricane's eye came ashore at about 7.15am local time close to Wilmington, with sustained winds of 150km/h, the National Hurricane Centre said.
By 1.50pm local time the winds had dropped to 120km/h and the centre was moving west at a 10km/h.
Parts of North and South Carolina are expected to get as much as one metre of rain.
As of Friday morning, Atlantic Beach, a town on the state's Outer Banks barrier islands, already had received 76 cm of rain, the US Geological Service said.
Authorities in New Bern said more than 100 people had to be saved from floods and that the downtown area was underwater.
The town's public information officer, Colleen Roberts, told CNN 150 more people were awaiting rescue.
'WE ARE COMING TO GET YOU,' New Bern city officials said on Twitter.
'You may need to move up to the second story, or to your attic, but WE ARE COMING TO GET YOU.'
Video reports from several towns in the Carolinas showed emergency personnel wading through rippling thigh-high floodwaters in residential neighbourhoods.
More than 634,000 homes and businesses were without power in North and South Carolina early on Friday, utility officials said.
It is expected to move across parts of southeast North Carolina and east South Carolina on Friday and Saturday, then head north over the western Carolinas and central Appalachian Mountains early next week.
Significant weakening is expected over the weekend.