Barack Obama pledges aid to the tornado hit communities, as he's set to travel to the worst-hit state of Alabama.
Shocked Americans are struggling to grasp the magnitude of the worst US tornadoes in decades, which carved a trail of destruction across the south, claiming at least 305 lives.
Communities like Alabama, Governor Robert Bentley's small hometown of Tuscaloosa, were virtually wiped off the map and officials warned the body-count would rise as rescuers uncover more dead amid the debris. The toll has reached 195 in Alabama alone.
Disbelief was written on faces across eight states crippled by the ferocious spring storms - the deadliest tornado tragedy to strike the United States since 310 people were killed in 1974.
Recalling the more recent horror of 2005's Hurricane Katrina, families picked through the remains of homes, businesses and schools, bearing witness to scenes of devastation more common in war zones or after earthquakes.
The toll reached 195 in Alabama and President Barack Obama said he would travel to the worst-hit state on Friday for a first-hand look at the unfolding human tragedy.
"The loss of life has been heartbreaking, especially in Alabama," Obama said at the White House, describing the disaster as "nothing short of catastrophic".
The storms "took mothers and fathers, sons and daughters, friends and neighbours - even entire communities," he said, vowing to rush federal assistance to those most in need.
States of emergency were declared from central Oklahoma to Georgia on the eastern seaboard and governors called out the National Guard - including 2,000 troops in Alabama - to help with the rescue and clean-up operations.
"We had a major catastrophic event here in Alabama with the outbreak of numerous long-track tornadoes," said Governor Bentley, who declared a major disaster for the state, where up to a million people were left without power.
Rescue workers search for survivors
Emergency responders searched through toppled trees, twisted billboards and ruined homes for signs of life, while thousands of survivors prepared to spend the night in makeshift shelters.
"There were direct hits on two towns, Hackleburg and Dadeville," National Weather Service (NWS) meteorologist in Alabama, John De Block, told AFP.
Most of the homes in Hackleburg, population 1,500, were built on slab foundations and "the slabs have been swept clean in several locations," he said, adding that colleagues told him the town was "90 per cent destroyed."
More than 160 tornadoes were reported on Wednesday alone and if more than 148 are confirmed that would set a new record for a 24-hour period, surpassing the so-called "Super Outbreak" in 1974.
Entire blocks obliterated in Tuscaloosa
In Tuscaloosa, population around 100,000, an AFP journalist witnessed entire blocks of the small town obliterated. In addition to 36 confirmed fatalities there were at least 600 people injured and dozens unaccounted for.
"Infrastructure has been absolutely devastated," Mayor Walter Maddox told CNN. "When you look at this path of destruction, likely five to seven miles (eight to 11 kilometers) long and half a mile to a mile wide, I don't know how anyone survived.
"There are parts of this city I don't recognise," he added.
It was also a dark day for Birmingham, Alabama's largest city with more than a million residents. Mayor William Bell spoke of "whole neighbourhoods of housing, just completely gone. Churches, gone. Businesses, gone".
Incredulous Birmingham residents assessing the damage were counting their blessings at having survived, while others were distraught over the loss of their loved ones or homes.
"There were two-by-fours (wooden beams) falling out of the sky," convenience store manager Jack Welch said, adding "there were well over 30 homes destroyed" just behind his store.
The overall toll includes 34 deaths in Tennessee, 32 in Mississippi, 14 in Georgia, 12 in Arkansas, five in Virginia, two in Missouri, and one in Kentucky, according to state officials contacted by AFP.
"Oh my God, our town is in pieces," said Tim Holt, a clerk at a local hotel in Ringgold, Georgia. "We saw the funnel cloud coming and I ran into the bathroom with my wife and daughter. It's an 80 per cent loss in our town."
Several eastern states were still on tornado and severe thunderstorm alert on Thursday, while another major storm system was forecast to bring heavy rain and high winds on Saturday.
