President Barack Obama offered federal assistance on Monday to stricken US states after powerful tornadoes killed 44 people and reduced whole neighborhoods to rubble.

Obama also put in a personal call to a North Carolina store owner who was credited with saving many lives before his massive building was reduced to a twisted heap of metal by a giant tornado.

"Today, the president called Michael Hollowell, manager of a Lowe's home improvement store devastated by a tornado Saturday in Sanford, North Carolina, to offer his gratitude for Mr. Hollowell's swift action that saved so many lives," a White House statement said.

Distraught residents and business owners across seven southern and central states, meanwhile, tried to piece their lives back together, particularly those in worst-hit North Carolina, home to exactly half those who died.

Thousands of residents were still in shelters or staying with relatives as officials surveyed scenes of utter devastation -- from obliterated buildings and upturned cars to flooded trailer parks and campsites.

North Carolina Governor Beverly Perdue, who described the disaster as the worst to hit her state since 1984, said federal officials had agreed to expedite the authorization of federal funds to help pay for the clean-up.

"It's not all about the money, but money helps," Perdue said.

The tornadoes first struck late Thursday in Oklahoma, where a giant twister almost flattened the small town of Tushka -- population 350 -- tearing up most of its homes and businesses and killing two elderly residents.

The storm system strengthened and expanded on Friday, whipping up hundreds more reported tornadoes that barreled through Arkansas, Mississippi, Alabama and North Carolina, before petering out in Virginia on Saturday night.

Falling trees snapped power lines and came crashing down on cars and houses, killing occupants and causing widespread damage. Witnesses described hailstones the size of grapefruit.

"The president spoke with Governor Perdue and (Alabama) Governor (Robert) Bentley yesterday to express his condolences and make sure they knew that the federal government stands ready through the Federal Emergency Management Agency to assist in the recovery efforts if needed," White House spokesman Jay Carney told a press briefing.

More than 30,000 homes and business in North Carolina were still without power and survivors clawed through the material and vegetative debris, separating it out and dragging it to the nearest roadside to be picked up.

"For those people who have insurance, they will be taken care of through their insurance companies," emergency management spokeswoman Patty McQuillan told AFP, explaining how the process worked.

"For those people with no insurance, they should take photos of the damage and go ahead and start their clean-up," McQuillan said.

"If the teams find 25 or more businesses in a particular county with uninsured losses that are more than 40-percent damaged, then the state will write a letter to the president asking for federal assistance."

Horrific tales were still coming to light.

In Raleigh, the North Carolina state capital, three young siblings died when a tree fell on their mobile home and hit the bathroom where they had taken refuge, police said.

Among seven people killed in Alabama were a mother and her two children sheltering inside their mobile home when it was thrown some 500 feet (150 meters) into the woods, landing on its roof.

Seven others died in Arkansas, five in Virginia, and one in Mississippi.

"There's no long-term idea of the economic damage at this point, people are still picking up pieces," Virginia emergency management spokeswoman Laura Southard told AFP.

"For the school that is heavily damaged they are trying to figure out how to complete the school year, they are not going to be able to return to that building. Many people are not able to return to their homes."

AccuWeather.com said a total of 267 tornadoes had been reported in 15 states over the three-day period, making it one of the largest tornado outbreaks in US history.

Experts said there had been three main contributory factors: a powerful jet stream, abundant moisture surging in from the Gulf of Mexico and a strong cold front plowing across the South.

The storms were the deadliest to strike the United States since tornadoes killed 57 people in southern and central states in February 2008.