As the death toll from Florence rose and hundreds were rescued from their flooded homes, North Carolina slid into the next stage of the disaster: catastrophic flooding.

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On Saturday Mitch Colvin, mayor of Fayetteville, told reporters: “The worst is yet to come.”

On Sunday, the death toll in North and South Carolina rose to 17; the city of Wilmington was cut off by floodwater; 15,000 people were in temporary shelters; and 760,000 North Carolinians remained without power.

“The storm has never been more dangerous than it is now,” Governor Roy Cooper said. “Many rivers are still rising, and are not expected to crest until later today or tomorrow.“

As controversy over the federal response to Hurricane Maria in Puerto Rico last year continued, Brock Long, director of the Federal Emergency Management Agency (Fema), told NBC’s Meet the Press staff were meeting challenges “as they’re coming up to us”. The focus, he said, was on search-and-rescue and helping those in shelters.

Donald Trump used Twitter to say Fema, “first responders and law enforcement are working really hard on hurricane Florence. As the storm begins to finally recede, they will kick into an even higher gear. Very Professional!”

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Nonetheless, the challenges were stiff. The mayor of New Bern, heavily hit by flooding, told NBC he had imposed a curfew. Thirty roads remained impassable, Dana Outlaw said, with 4,200 homes and more than 300 commercial buildings damaged, 6,000 people without power and 1,200 in shelters.

On Saturday night, Duke Energy said heavy rains caused a slope to collapse at a coal ash landfill at a closed power station outside the port city of Wilmington. A Duke spokeswoman said about 2,000 cubic yards of ash were displaced at the Sutton Plant and contaminated water probably flowed into the cooling pond.

Ash left when coal is burned contains toxic heavy metals, including lead and arsenic. The company had not determined if the weir that drains the pond was open and if contamination may have flowed into the Cape Fear river.

This is not a talking point. We are saying this because we are concerned with you. The worst is yet to come Mitch Colvin, mayor of Fayetteville

After blowing ashore as a hurricane with 90mph winds, Florence lingered over the Carolinas. By Sunday it had weakened to a tropical depression. Winds were down to 35mph. But in North Carolina, rivers were swelling.

The evacuation zone included part of Fayetteville. “This is not a talking point,” said its mayor, Colvin, on Saturday. “This is not a script, but we are saying this because we are concerned with you. The worst is yet to come. If you are refusing to leave during this mandatory evacuation, you need to do things like notify your legal next of kin. The loss of life is very, very possible.”

Forecasts said rivers would crest on Sunday and Monday at record or near-record levels: the Little, the Cape Fear, the Lumber, the Neuse, the Waccamaw and the Pee Dee were all projected to burst their banks.

In Goldsboro, rain fell and the Neuse was swelling above its banks. Sunday service at the Greenleaf Christian church was cancelled. The church’s pastor is the civil rights leader William Barber. He was forced to evacuate his 86-year-old mother west, to Greensboro.

Speaking on the phone from Washington, Barber told the Guardian he planned to return to oversee outreach efforts. The church, which serves some of the poorest residents, will hand out meals to children who would usually receive food at school.

He said: “I have members who live in rental housing that were afraid their housing was going to get destroyed during this particular hurricane. Thank God it lowered down to a category one. If it were a category three or four a number of my members would be out of home.”

Play Video 1:04 Storm Florence: footage shows scale of flooding in North Carolina – video

In Lumberton, the river is considered flooded at 13ft. The National Weather Service predicted on Saturday it would crest at 24.9ft on Sunday. On Saturday, much of the south of the city was already underwater.

Turner Park, a trailer park, was under 2ft. The Guardian watched as water crept towards Martin Luther King Jr Drive. Residents had been told to evacuate the park, where scores of 40ft x 12ft trailers stand on low-lying land. Across the road Newport church, a handsome white building with an impressive steeple, was under about a foot of water. Further north, single-storey homes were flooded.

Twenty-five per cent of the population of Lumberton lives below the poverty line. The city was hit hard by Hurricane Matthew two years ago. Residents face losing property and possessions all over again. Shelters in nearby Fayetteville were full.

On Sunday, about three miles south of the city, the coast guard went door to door – mostly by boat – urging people to leave.

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In Wilmington, media said water supplies could be turned off. Families discussed finding batteries for generators and pouring drinking water into coolers.

A mother and baby in Wilmington were among the dead. Three people died in one inland county, Duplin, because of flooding. A husband and wife died in a storm-linked house fire, officials said, and an 81-year-old man died after falling while packing to evacuate.

In South Carolina, a 61-year-old woman was killed when her car hit a tree on a highway. Authorities said a 63-year-old man and a 61-year-old woman were asphyxiated after using a generator inside their home. The overall toll climbed to 16 after a driver lost control and went off the side of a road near Gilbert, hitting a tree.

The White House said Trump “continues to monitor the preparedness and response efforts” and had been briefed on Sunday afternoon.

But the president’s tweets about another storm, Maria, continued to cause controversy. Trump has repeatedly claimed the recognised death toll, around 3,000, has been inflated by his political opponents.

Alexandria Ocasio-Cortez, the New York congressional candidate who has become a leading voice on the progressive left, told CNN’s State of the Union Puerto Ricans, subject to the “worst humanitarian catastrophe in modern American history”, pointed to “government inaction as the cause of death”.