More than 60 tornadoes ripped through North Carolina, killing 21, the highest toll for any of the states affected.
A White House spokesman said the government stood ready to provide aid.
Governor Beverly Perdue said on the NBC network's Today show that she had never seen anything like the devastation.
She said homes in the state had been handled like paper dolls' houses.
"The good news is that the tornadoes have left and things are brighter today in North Carolina," Ms Perdue said.
At the White House on Monday, spokesman Jay Carney said the Federal Emergency Management Agency was monitoring conditions.
"We have folks on the ground assessing the situation and standing ready to provide assistance as necessary," he said.
The killer storms cut across a vast swath of the American south, sweeping east across Oklahoma and Arkansas on Thursday and Friday then through Mississippi and Alabama and up through Virginia and North Carolina.
The storms, which also caused damage in the states of Texas and Georgia, moved out over the Atlantic on Sunday.Devastating destruction
More than 240 tornadoes were reported over the three days, including 62 in North Carolina, but the US National Weather Service's final numbers could be lower because some tornadoes may have been reported more than once.More than 60 tornadoes were reported in North Carolina, where officials confirmed 21 deaths
The North Carolina state emergency management agency said it had reports of 23 deaths from Saturday's storms, but local officials have only confirmed 21.
Authorities in the city of Raleigh early on Monday were blocking access to a mobile home park of roughly 200 homes, where three children had been killed during the storms.
Ms Perdue said she planned to tour hard-hit areas in three counties in the state on Monday.
She added that the devastation she had seen on Sunday had left her in tears.
The governor said she had contacted President Barack Obama, who pledged his support, and that federal emergency workers had already been deployed to the state.
"We have in North Carolina a tremendous relationship with our federal partners, and have been through this so many times," she said.
"That's not a good thing. That's a bad thing," she added.Ms Perdue said on Sunday the number of tornadoes had been the highest since 1984, when tornadoes killed 42 people.
Hailstones the size of grapefruit were reported as the storms swept through the region, causing flash floods as well as tornadoes.
Trees and downed power lines still covered nearby roads on Monday.
Most of North Carolina's 21 confirmed deaths occurred in two rural counties - 11 in Bertie and four in Bladen, about 70 miles (112 km) south of Raleigh.
In the Bladen County community of Ammon, Audrey McKoy and her husband, Milton, witnessed a tornado lifting pigs and other animals into the sky, as the storm struck their mobile home community.
"It looked just like The Wizard of Oz," Mrs McKoy said.
Mr McKoy found three bodies in their neighbourhood after the storm, which spun their mobile home, had passed.
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