Storm stalks southeastern America

Catastrophic floods raised the threat of landslides and dam failures across the southeastern United States on Monday, prolonging the agony caused by a killer hurricane that has left more than a dozen people dead and caused billions of dollars in damage.

Downgraded to a tropical depression, Florence crept over South and North Carolina, dumping heavy rains on already flood-swollen river basins that authorities warned could bring more death and destruction.

The National Hurricane Centre said the storm was expected to weaken further on Monday "before re-intensifying as it transitions to an extratropical cyclone Tuesday and Wednesday." The National Weather Service Weather Prediction Centre warned of "heavy and excessive rainfall over the next couple of days."

There is an "elevated risk for landslides" in western North Carolina and southwest Virginia, as well as a danger of "catastrophic and life-threatening flash flooding" in parts of the Carolinas, it said.

"A lot of people have evacuated already," said Denise Harper, a resident of Grifton, a small North Carolina town threatened by rising water levels in a nearby creek and the River Neuse. "It’s worrying to watch the water slowly rising."

At least 15 people have died since Florence made landfall Friday as a Category 1 hurricane near Wrightsville Beach -- 10 in North Carolina and five in South Carolina. "Unfortunately we’ve still got several days to go," Brock Long, administrator of the Federal Emergency Management Agency, told Fox News.

Long said more havoc lies ahead as the storm broadens its geographic scope over regions deeply saturated with water. Of particular concern were the risks to dams, already stressed by heavy rainfall from a tropical storm earlier in the month, he said, urging citizens to heed official warnings about what is now a "flood event."

"What we have to focus on (is whether there) are any dams that are potentially going to break." "People fail to heed warnings and get out or they get into the floodwaters trying to escape their home. And that’s where you start to see deaths escalate," he told CBS News. "Even though hurricanes are categorized by wind, it’s the water that really causes the most loss of life."