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. 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.