A killer storm that left up to 13 people dead weakened to a tropical depression on Sunday, but US authorities warned the devastation it caused -- including catastrophic flooding -- is far from over.

Most of the fatalities from Florence, which made landfall Friday as a Category 1 hurricane, have occurred in North Carolina, where officials confirmed eight victims. They included three who died "due to flash flooding and swift water on roadways," the Duplin County Sheriff's Office reported.

A woman and her baby were among the first casualties when a tree fell on their house, contributing to a death toll that US media said had reached 13 -- 10 in North Carolina and three in South Carolina, according to CNN.

The National Hurricane Center (NHC) downgraded Florence to a tropical depression on Sunday morning, adding that "flash flooding and major river flooding will continue over a significant portion of the Carolinas."

As of 5:00am (0900 GMT) yesterday, maximum sustained winds had weakened to near 35 miles (56 kilometers) per hour, the NHC said.

On Saturday, some residents tried to return home, driving through flooded highways and armed with chainsaws to clear fallen pine trees that covered the road.

North Carolina Governor Roy Cooper warned against such behavior as roads became increasingly dangerous.

Steve Goldstein of the National Oceanic and Atmospheric Administration (NOAA) said some areas have already received two feet of rain and could expect up to 20 inches more as the system moved "slowly, nearly stationary" over eastern North Carolina.

South Carolina Governor Henry McMaster on Saturday announced that a 61-year-old woman had died when her car hit a downed tree. CNN also reported a man and a woman died in South Carolina's Horry County due to carbon monoxide poisoning.

More than 800,000 in North Carolina were without power and 21,000 people were being housed in 157 shelters across the state.

The military announced Saturday it was deploying nearly 200 soldiers to assist in storm response and recovery efforts, along with 100 trucks and equipment.

Hurricane Florence made landfall Friday in Wrightsville Beach, North Carolina, after stalking the coastline for days.

Tornadoes remain a threat, with the NHC saying that "a few tornadoes remain possible across North Carolina and eastern South Carolina."