President Obama walked through a tornado-ravaged neighborhood in Tuscaloosa on Friday and promised “maximum federal help” to the survivors of a series of deadly twisters that carved paths of destruction and claimed about 300 lives in six Southern states.

“I’ve never seen devastation like this,” Obama said as he toured the Alberta section of the city with first lady Michelle Obama and gazed at crumpled houses, uprooted trees and destroyed cars. “It is heartbreaking.”

The powerful tornadoes left stunned residents literally picking up the pieces Friday, as they sought to salvage what they could in shattered homes from Mississippi to Virginia.

It was the nation’s deadliest natural disaster since Hurricane Katrina — a “tornado outbreak” rarely seen on such a scale.

Obama, who on Thursday called the damage “nothing short of catastrophic,” flew to Alabama with his wife Friday morning to view the devastation in hard-hit Tuscaloosa and meet with Gov. Robert Bentley (R) and affected families. The Obamas later took off for Cape Canaveral, Fla., where they had planned to witness the launch of the space shuttle Endeavour in the afternoon. The launch was scrubbed around midday because of mechanical problems, but Obama went ahead with his trip there.

Obama signed a “major disaster” declaration for Alabama late Thursday, making federal aid available to supplement state and local recovery efforts.

Speaking to reporters against a backdrop of splintered houses in the Tuscaloosa neighborhood, Obama said his administration would “make sure the maximum federal help comes here as quickly as possible.”

Standing in shirtsleeves in bright sunshine and flanked by Bentley and Tuscaloosa Mayor Walter Maddox, Obama said: “We are going to do everything we can to help these communities rebuild. We can’t bring those who’ve been lost back — they’re alongside God at this point. . . . But the property damage, which is obviously extensive, that’s something that we can do something about.”

He told Maddox, “We’re going to make sure you’re not forgotten.”

Bentley said 210 Alabama residents were confirmed dead in the disaster, 1,700 were injured and “a number of people” were still missing.

At least 34 people were killed in Mississippi, 34 in Tennessee, 15 in Georgia, five in Virginia and one in Kentucky, state officials reported. Many more were injured.

In Alabama, emergency management officials said that with search and rescue operations ongoing, it was too early to conduct total damage assessments.

“It’s hard to think on Friday that we’ll have a dollar amount that soon,” said Yasamie August, a spokeswoman with the Alabama Emergency Management Agency. Although several states have volunteered machinery and manpower if necessary, the state hasn’t accepted any assistance so far, she said.

In neighboring states, officials hailed the federal government’s response — especially preparations conducted by FEMA in anticipation of formal requests for aid.

“Anything that we’ve asked for, they’ve gotten us,” said David Maxwell, director of the Arkansas Department of Emergency Management. His state expected to issue a request for federal assistance as it recovers from the tornadoes and separate flooding incidents in northern portions of the state.

Maxwell said FEMA Administrator W. Craig Fugate and his deputies “are proactive, and they’re communicating with us regularly.”

Not since April 3, 1974, has the United States witnessed so much destruction from twisters, and tornado experts say Wednesday’s outbreak may go down in history as the most destructive in eight decades.

Alabama took the most brutal pounding, the state scarred by a monster funnel cloud that crossed the state on a track that struck Tuscaloosa head-on and chewed through the Birmingham suburbs before exiting into Georgia.

As many as 1 million homes and businesses in Alabama lost power, and Bentley activated 2,000 National Guard troops to help in the recovery effort.

The National Weather Service on Friday upgraded the severity of a tornado that struck the small town of Smithville (population about 860) in northeastern Mississippi on Wednesday afternoon, killing 14 people and injuring 40. The half-mile-wide twister, which stayed on the ground for nearly three miles, packed peak winds of 205 mph, making it a rare category EF-5 tornado, the weather service said.

The rating on the service’s Enhanced Fujita Scale is the highest indicator of wind speed and related damage. The Smithville tornado, the most powerful to hit Mississippi since 1966, destroyed or damaged dozens of homes and businesses, including the post office and police station, and wrecked the town’s water system. Most trees were snapped or twisted, and one 1965 Chevy pickup that was parked in front of a destroyed home has not been found, the weather service said.

During a visit to Smithville, Mississippi Gov. Haley Barbour (R) expressed fears that the state’s death toll may rise.

“We know there is a tremendous amount of debris” that could be covering additional bodies, he told reporters. He said there was also “some risk that the waterways that surround this area could possibly contain human remains.”

Even as survivors combed the wreckage of their homes for recoverable belongings, search-and-rescue crews continued Friday morning to look for victims of the disaster.

In Virginia, rescuers rushed a survivor to a hospital after pulling the person out of rubble Friday morning in Washington County in the southwestern part of the state, emergency officials said. The rescue came a day after an EF-3 tornado roared through the area with winds up to 165 mph.

In Tuscaloosa, home to the University of Alabama, a team with a cadaver-sniffing dog searched the wreckage of an apartment complex reduced to rubble by the tornado.

Maddox, the Tuscaloosa mayor, said entire neighborhoods were wiped off the map. The city of more than 80,000 lost its emergency management agency and one of its fire stations, Maddox said. In addition, most of its garbage and recycling trucks were damaged or destroyed, and two major water tanks were out of water, he said.

“This place looks like a war zone,” Jackie Wuska Hurt, director of development for the honors college at the University of Alabama, wrote in an e-mail Thursday. “Folks looked like refugees walking single file with suitcases or grocery carts of their belongings down the sidewalks of University Boulevard.”

“It’s almost total disbelief,” said Phyllis Little, director of emergency management for Cullman County, Ala., a largely rural area of 82,000 peppered with small towns. “The county courthouse lost its roof. The baptist church has a skeleton for a steeple. Old buildings that have been there for hundreds of years have just collapsed.”

The entire county was without power, and emergency responders were operating on natural gas generators. Little has been turning away volunteers who have called her office, offering to come to Cullman to help.

“Fuel is an issue for us,” Little said. “We’re struggling to provide that to the emergency response agencies. If you don’t live here or have business here, don’t come.”

‘Little middle ground’

Local TV stations in Alabama captured stunning footage of the squat, black maelstrom as it chewed a path through Tuscaloosa shortly before dusk Wednesday, riding along an interstate highway and coming within a mile of the football stadium that is home to the fabled Crimson Tide.

The university has closed, canceling final exams and postponing graduation exercises until August. Power outages shut down most forms of communication, but students found they could still track the news through Twitter.

“Somehow the Twitter feeds keep coming,” said Ian Sams, 22, a senior. “You’d see people tweeting from shelters saying. ‘We need blankets. We need diapers. If you can bring them, bring them.’ ”

As with any tornado, the destruction could seem capricious, with obliterated areas bracketed by neighborhoods that were merely a little windblown.

“There’s very little middle ground. Either you took a beating, like you really were just devastated by it, or I went to my parents’ house, and they have power, and it’s just another day,” said Brandi Freeman, 21, a senior.

Alabama’s Emergency Management Agency said 31 of the state’s 67 counties have reported damage. Most are in the central and northern parts of the state.

“This was the big one,” said James-Paul Dice, chief meteorologist at WBRC Fox 6 in Birmingham. “A monster of a storm.”

Dice said the biggest tornado passed two miles from the station as most of his co-workers took shelter. He continued broadcasting, telling his viewers that this was unlike anything he’d seen in his 16 years in the business.

That this would be a day of severe storms had been known many days in advance, thanks to computer models of the weather pattern, but Dice said he was shocked Wednesday morning at some of the numbers he was seeing. He said there is a measure of potential tornadic activity known as the “energy helicity index.” Anything in the range of 3 or 4 would suggest a possible tornado, and he was stunned to see, on Monday, a forecast of a 6 for Wednesday. Then, Wednesday morning, the index jumped to 14.

“It was off the charts. This was almost like made-up numbers,” Dice said.

Meteorologists are on the ground examining the damage in an attempt to get a precise handle on the number of distinct tornadoes and their intensity. What seems certain is that this was the worst day for twisters in America since Richard M. Nixon was in the White House.

“The outbreak is the biggest in terms of tornadoes and in terms of impact since ’74, and it’s possible that it’s actually bigger than ’74,” said Harold Brooks, research meteorologist at the National Severe Storms Laboratory in Norman, Okla.

The April 3, 1974, outbreak sparked twisters across the eastern United States, claiming 310 lives, Brooks said. Wednesday’s outbreak may be most similar to the tornado outbreak of March 21, 1932, when 332 people were killed, including 268 in Alabama, he said. Nothing, however, comes close to the destruction of March 18, 1925, when 747 people died, most of them along the path of a single twister, the so-called Tri-State Tornado that tore up Missouri, Illinois and Indiana.

Brooks said the conditions have been ripe in recent weeks for just such a catastrophe. Cold, dry air aloft, powered by the jet stream, blows in from the west, meeting the low-level, warm, moist air moving northward from the Gulf of Mexico. If the cold fronts are strong enough, they’ll suppress tornado formation. But if they’re weak, the result can be a deadly compromise between the colliding air masses: The warm air at ground level will be moving in a different direction from the air higher up. That’s a recipe for the rotational energy that spawns a full-blown tornado.

At the nuclear plant

The storms shut down the three nuclear reactors at the Browns Ferry power plant 30 miles west of Huntsville, Ala., a plant of similar design to the severely damaged Fukushima Daiichi power plant in Japan. But unlike Fukushima Daiichi, when Browns Ferry lost primary power, the plant’s diesel generators kicked in as designed to keep the reactors cool, said Barbara Martocci, a spokeswoman for the Tennessee Valley Authority, which operates the 3,274-megawatt facility. “The plant is shut down safely,” she said, meaning that control rods dropped into the reactors when power went offline, stopping nuclear fission.

The plant’s cooling systems can run indefinitely on diesel generators as crews work to restore external power, Martocci said. The facility will not begin producing electricity again until “we have a full damage assessment of our entire transmission system,” Martocci added.

In Virginia, five people died — three in the small town of Glade Spring — in Washington and Halifax counties, when twisters roared through overnight, officials said.

A truck stop on Interstate 81 and a new factory were destroyed, according to Christy Parker, assistant administrator in Washington County, in southwest Virginia.

Tractor trailers “were flipped and thrown about the interstate like toys,” she said Thursday.

Pokey Harris, Washington County’s director of emergency management, said late Thursday: “We have multiple injuries . . . broken bones, crush injuries. We have a tremendous amount of devastation. A lot of buildings are destroyed.”

Most of the Virginia fatalities occurred when what appeared to be a tornado hit a mobile home park, the truck stop and an apartment complex, Virginia Department of Emergency Management officials said. Storms ripped through a subdivision in Shenandoah County, damaging several homes. Virginia Gov. Robert F. McDonnell (R) declared a state of emergency, authorizing state agencies to assist local governments in responding to the impact of the weather.

Severe storms and possible tornadoes also struck Goochland County, officials said. Fifty people were injured statewide.

Tornado warnings were issued across the D.C. suburbs early Thursday morning. Fast-moving bands of storms packed high winds and torrential rains. A funnel cloud formed over Point of Rocks in Frederick County shortly before 7 a.m., according to the National Weather Service.

There were widespread reports of damaged trees, including one that fell across Route 109 in Barnesville and another that landed on an electric line in Middleburg, according to the weather service. Fauquier County schools were delayed by two hours, and Prince William County schools canceled outdoor activities.

The storms caused flight delays of up to 90 minutes at Reagan National and Dulles International airports.

O’Keefe and Branigin reported from Washington. Staff writers Joel Achenbach, Michael Bolden, Michael E. Ruane, Jason Samenow, Krissah Thompson, Brian Vastag and Erin Williams and researcher Madonna Lebling in Washington contributed to this report.