US tornadoes: Barack Obama shocked by devastation
US President Barack Obama has said he has "never seen devastation like this" after visiting a tornado-ravaged city in the south-eastern state of Alabama.
Speaking in Tuscaloosa, Mr Obama said Washington would do everything possible to help communities rebuild.
At least 318 people died, most of them in Alabama, as tornadoes ripped through a swathe of states.
Widespread devastation has also been reported in Tennessee, Mississippi, Georgia and Virginia.
A state of emergency has been declared in seven states, and federal aid money is being sent to Alabama.
"We're going to make sure you're not forgotten," Mr Obama told Tuscaloosa survivors as he and his wife, Michelle, toured the devastated city.
A twister thought to be a mile wide (1.6km) ploughed through the city earlier this week.
As many as a million homes and businesses across Alabama are still without power.
The US National Weather Service (NWS) has reports of nearly 300 tornadoes since the storms began a week ago, more than 150 of them on Wednesday alone.
"These were the most intense super-cell thunderstorms that I think anybody who was out there forecasting has ever seen," Greg Carbin of the NWS Storm Prediction Center in Oklahoma told the Associated Press news agency.
The NWS also said one tornado in Mississippi on Wednesday had wind speeds of 205mph (330 km/h) - the first EF-5 tornado in that state since 1966, the Associated Press news agency reports.
This is the highest rating given by the NWS for tornado damage.
In Alabama, 2,000 soldiers joined emergency workers as they continued to search for survivors.
Many people were picking through the remains of destroyed homes, trying to salvage what they could.
Alabama Governor Robert Bentley said he expected the death toll to rise as more bodies were discovered.
The number of deaths from the recent series of tornadoes is the greatest in the US since 1974, when a sequence of twisters claimed 315 lives in 13 US states.
Tuscaloosa Mayor Walter Maddox said after an aerial tour: "We have neighbourhoods that have been basically removed from the map."
An aide to the mayor, Heather McCollum, told Reuters news agency that cadaver dogs were being brought in on Friday to help search the wreckage.
One Tuscaloosa resident, Angela Smith, whose neighbour was killed, told Reuters: "I made it. I got in a closet, put a pillow over my face and held on for dear life because it started sucking me up."
Another town, Hackleburg, was reported to be "90% destroyed".
The mayor of Birmingham, William Bell, said "whole neighbourhoods of housing, just completely gone. Churches, gone. Businesses, gone... [it] seems like a bomb has been dropped".
More than 25 people died in Phil Campbell, a town of about 1,000 in north-west Alabama.
Jerry Mays, the town's mayor, said the tornado that destroyed the town's grocery store and medical clinic was a half-mile wide and travelled for about 20 miles (32km).
"We've lost everything. Let's just say it like it is," Mr Mays said.
Hundreds of thousands of homes have lost power in Alabama after a nuclear plant automatically shut when its power was knocked out by the storms, although back-up systems worked, said officials.
Mississippi reported 33 deaths on Tuesday and Wednesday. In Smithville, Mississippi, many buildings were ripped open, including a church, the city hall and the post office.
At least 15 people have been killed in Georgia and five in Virginia.