The death toll approached 300 tonight from the tornadoes and thunderstorms that tore through the South Wednesday night and early today.
NOAA, the National Oceanic and Atmospheric Administration, estimated there were 173 tornadoes Wednesday, a new record for a single storm system in modern times.
President Obama called the loss of life "heartbreaking," and promised those affected by the storms the full support of the federal government.
"We can't control when or where a terrible storm may strike, but we can control how we respond to it," Obama said during a press conference today at the White House. "I want every American that's been affected by this disaster to know the federal government will do everything we can to help you recover and we will stand with you as you rebuild."
The twisters rampaged through cities like Tuscaloosa, Ala., forced a pair of nuclear plants to go off line, left thousands homeless and more than a million people without power.
NOAA said it was the worst tornado outbreak since 1974, when storms killed 315 people. The deadliest tornado outbreak on record was on March 18, 1925, when 695 people died.
"In a matter of hours, these deadly tornadoes ... took mothers and fathers, sons and daughters, friends and neighbors, even entire communities," Obama said.
Georgia Gov. Nathan Deal toured the tornado area this morning and said, "It's hard to imagine nature can be this devastating."
The governor said it was a remarkably wide swath of twisters and noted there were multiple tornadoes in some areas.
The majority of the deaths reported so far have been in Alabama, where at least 195 people were killed. Mississippi recorded 32 fatalities, Tennessee had 33, Georgia at least 10, and Virginia had eight.
Hundreds more were injured. Of the 134 patients brought to the University of Alabama hospital in Birmingham, 40 were major trauma injuries, the hospital said.
Loring Rue, M.D., chief of trauma surgery at UAB Hospital, said 134 patients came through the E.R.; 40 were major trauma injuries and 23 were admitted to intensive care units. In addition,10 surgical procedures were performed.
"The injuries were remarkable," Rue said, adding that people who were in the comfort of their homes when the tornado struck were brought in with injuries doctors normally see in high-speed motor-vehicle accidents
Greg Carbin, warning coordination meteorologist at the National Weather Service, said the area can expect mild weather for the next few days but he could not predict whether more killer tornadoes were on their way.
"May is usually the most active tornado month," Carbin said. "Will it maintain the activity of April? We just cannot say."
President Obama called Alabama Gov. Robert Bentley to express condolences over the deaths in his state and the White House announced that the president will stop in Alabama Friday to tour the wreckage while on his way to Cape Canaveral for the launch of the space shuttle Endeavour.
Possibly the hardest-hit area was Tuscaloosa, where at least 32 people died. Early estimates indicated that the tornado that hit Tuscaloosa could have been on the ground for 176 miles, with winds between 167 and 200 mph.
The city was stunned by the devastation that not only hit homes and commercial areas, but also destroyed much of the city's public works infrastructure -- including the city's emergency management administration headquarters.
"We are critically short of men, material and equipment," Mayor Walter Maddox said at a news conference today.
The mayor asked people to stay off the streets and conserve water, and for gawkers to stay away. He said "sightseers" are only getting in the way of emergency crews.
"This is going to be a very, very long process" of cleaning up and rebuilding, he said. "During this time we ask for patience and we ask for prayers."
About 2,000 Alabama National guard soldiers have been activated to help with search and rescue.
"There is some massive devastation out there," Gov. Bentley said. "We have some people that are hurting."
Two nuclear power plants in Browns Ferry, Ala., had to be shut down after the storm damaged its transmission system. Emergency generators kicked in to cool down the reactors safely.
In Ringgold, Ga., near the Tennessee border, officials shut down major roads leading into the city because of what Catoosa County Sheriff Phil Summers said was "downed power lines, broken gas lines and looting."
"There are lots of spectators and we are not allowing access inside of Ringgold at this time," the sheriff said.
Summers said he saw the tornado in the air just before it touched down.
"It was something that I've never seen before except on TV. And like they say, there's a moment of silence and then you see the devastation," he said.
The sheriff said the tornado was in Ringgold for about five minutes and it continued into Tennessee. Five people died and police are looking for others whose homes were so badly damaged that "only foundations are left," Summer said.
Christopher England at the University of Alabama ran up to the roof of his building to videotape a devastating tornado as it hit Tuscaloosa.
"We just saw this massive huge mile wide tornado and we didn't know where it was going. We didn't know if it was coming towards us or away from us or what," he said.
One of the victims was an off-duty police officer in northern Mississippi who died while shielding his daughter from a falling tree on a camping trip. She wasn't hurt.
ABC News Radio, Michael S. James, Mike Marusarz, Jessica Hopper and the Associated Press contributed to this report.