It diminished from hurricane force as it came ashore, but forecasters said the 560km-wide storm's slow progress across North and South Carolina could leave much of the region under water in the coming days.

"This storm is relentless and excruciating," North Carolina Governor Roy Cooper told CNN late on Friday.

"There is probably not a county or a person that will not be affected in some way by this very massive and violent storm."

A mother and baby were killed when a tree fell on their home in Wilmington, North Carolina. The child's injured father was taken to hospital. In Pender County, a woman died of a heart attack; paramedics trying to reach her were blocked by debris.

Two people died in Lenoir County. A 78-year-old man was electrocuted attempting to connect extension cords while another man died when he was blown down by high winds while checking on his hunting dogs, a county spokesman said.

In New Bern, North Carolina, the storm surge overwhelmed the town of 30,000 which is located at the confluence of the Neuse and Trent rivers.

Officials in New Bern, which dates to the early 18th century, said more than 100 people were rescued from floods and the downtown was under water by Friday afternoon.

Resident Jay Manning said he and his wife watched with alarm as water filled the street.

"We moved all the furniture up in case the water comes in but the water seems to be staying at the edge of the driveway," he said, adding that if the wind picks up and the rain keeps coming, that could change. "My wife's in a panic right now."

Florence was a Category 3 hurricane but was downgraded to Category 1 before coming ashore close to Wilmington, North Carolina.

The National Hurricane Centre (NHC) downgraded it to a tropical storm on Friday, but warned it would dump as much as 76-102cm of rain on the southeastern coast of North Carolina and part of northeastern South Carolina.

About 10 million people could be affected by the storm.

More than 22,600 people were housed in 150 shelters statewide.

"This rainfall will produce catastrophic flash flooding and prolonged, significant river flooding," the hurricane centre said.

North Carolina utilities estimated that as many as 2.5 million state residents could be left without power, the state's Department of Public Safety said.

The White House said on Friday that President Donald Trump had spoken with state and local officials, assuring them the federal government was prepared to help. Trump plans a visit to the region next week.

The storm is expected to turn west and then north moving through the Carolinas and the Ohio Valley by Monday, the NHC said early on Saturday. Significant weakening was expected over the weekend.

Australian Associated Press