Oklahoma residents are being told to brace for more fatalities after a massive tornado cut through suburbs this morning, killing 91 people.
A massive rescue operation is under way but the Oklahoma Medical Examiner's Office said 20 children were among the dead.
The twister, which stretched 3.2km-wide at times, made a direct hit on a school and pulverised entire neighbourhoods and set buildings on fire.
The ferocious storm, with winds of up to 320kmh - less than 1 per cent of all tornadoes reach such wind speed - struck the suburb of Moore, sending houses, vehicles and debris flying into the air.
The National Weather Service Storm Prediction Center provided the town with a warning 16 minutes before the tornado touched down at 3.01pm (local time), which was greater than the average eight to 10 minutes of warning, said Keli Pirtle, a spokeswoman for the centre in Norman, Oklahoma.
President Barack Obama declared a major disaster area in Oklahoma, ordering federal aid to supplement state and local efforts.
The death toll was expected to rise and officials said the clean up would take many days or weeks.
Plaza Towers Elementary School was left a pile of rubble. At least 75 students were inside at the time. They were told to "hug the walls" as they braced for the winds.
Rescuers swarmed the debris as darkness fell. More than 145 people were being treated at hospitals, including about 70 children. Some were in critical condition.
"Hearts are broken" for parents looking for their children, Governor Mary Fallin told a news conference.
Two dozen were missing in the rubble of the school, Oklahoma Lieutenant Governor Todd Lamb said.
"I have never seen anything like this in my 18 years covering tornadoes here in Oklahoma City. This is without question the most horrific," said Lance West, a reporter for KFOR.
Dogs have been called in to help the search effort and cries for help could be heard from the debris.
"The whole city looks like a debris field," Glenn Lewis, the mayor of Moore, told NBC. "It looks like we have lost our hospital. I drove by there a while ago and it's pretty much destroyed," Lewis said.
Spotlights bore down on massive piles of shredded cinder block, insulation and metal as crews worked through the night lifting bricks and parts of collapsed walls. Rescuers walked through neighbourhoods, where home after home was flattened and trees stripped of their leaves, to see if they could hear any voices calling out from the rubble.
The storm laid waste to scores of buildings in the community of 41,000 people, which lies about 16km south of Oklahoma City. Block after block lay in ruins. Homes were crushed into piles of broken wood. Cars and trucks were left crumpled on the roadside.
The National Weather Service estimated the tornado was an EF-4 on the enhanced five-point Fujita scale, the second most powerful type of twister.
Families anxiously waited at nearby churches to hear if their loved ones were OK. A man with a megaphone stood Monday evening near St. Andrews United Methodist Church and called out the names of surviving children. Parents waited nearby, hoping to hear their sons' and daughters' names.
Amy Elliott, spokeswoman for the Oklahoma Medical Examiner's Office, told The Associated Press early Tuesday that officials could see as many as 40 more fatalities from the tornado in addition to the 51 already confirmed dead. She said at least 20 children were among the confirmed dead.
Reports have said 91 people have died so far.
In video of the storm, the dark funnel cloud could be seen marching slowly across the green landscape. As it churned through the community, the twister scattered shards of wood, awnings and glass all over the streets.
National Guard troops were deployed by the governor to assist with rescue operations and activated extra highway patrol officers.
Fallin also spoke with Obama, who declared a major disaster and ordered federal aid to supplement state and local recovery efforts.
'HELP, HELP. MY HOUSE IS ON TOP OF ME'
At Plaza Towers Elementary School, the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.
Children from the school were among the dead, but several students were pulled alive from the rubble. Rescue workers passed the survivors down a human chain to the triage centre in the parking lot.
James Rushing, who lives across the street from the school, heard reports of the approaching twister and ran to the school, where his 5-year-old foster son, Aiden, attends classes. Rushing believed he would be safer there.
"About two minutes after I got there, the school started coming apart," he said.
The students were sent into the restroom.
Don Denton hadn't heard from his two sons since the tornado hit the town, but the man who has endured six back surgeries and walks with a severe limp said he walked about 3km as he searched for them.
As reports of the storm came in, Denton's 16-year-old texted him, telling him to call.
"I was trying to call him, and I couldn't get through," Denton said.
Eventually, Denton said, his sons spotted him in the crowd. They were fine, but upset to hear that their grandparents' home was destroyed.
As dusk began to fall, heavy equipment was rolled up to the school, and emergency workers wearing yellow crawled among the ruins, searching for survivors.
Because the ground was muddy, bulldozers and front-end loaders were getting stuck. Crews used jackhammers and sledgehammers to tear away concrete, and chunks were being thrown to the side as the workers dug.
Many land lines to stricken areas were down, and cellphone networks were congested. The storm was so massive that it will take time to establish communications between rescuers and state officials, the governor said.
Tiffany Thronesberry said she heard from her mother, Barbara Jarrell, shortly after the tornado struck.
"I got a phone call from her screaming, 'Help. Help. I can't breathe. My house is on top of me'," Thronesberry said.
Thronesberry hurried to her mother's house, where first responders had already pulled her out. Her mother was hospitalised for treatment of cuts and bruises.
The tornado also destroyed the city hospital and numerous businesses. Moore Mayor Glenn Lewis watched it pass through from his jewellery shop.
"All of my employees were in the vault," Lewis said.
Lewis, who was also the mayor of Moore when the strongest tornado on record whipped the city in 1999, said the most recent storm won't deter the community from rebuilding.
'A LOW ROAR'
Chris Calvert saw the menacing tornado from about a mile away.
"I was close enough to hear it," he said. "It was just a low roar, and you could see the debris, like pieces of shingles and insulation and stuff like that, rotating around it."
Even though his subdivision is a mile from the tornado's path, it was still covered with debris. He found a picture of a small girl on Santa Claus' lap in his yard.
A map provided by the National Weather Service showed that the storm began west of Newcastle and crossed the Canadian River into Oklahoma City's rural far southwestern side about 3pm. When it reached Moore, the twister cut a path through the centre of town before lifting back into the sky at Lake Stanley Draper.
Oklahoma City Police Captain Dexter Nelson said downed power lines and open gas lines posed a risk in the aftermath of the system.
The powerful tornado loosely followed the path of a killer twister that slammed Moore in May 1999. That storm produced the highest winds ever recorded near the Earth's surface - 302 mph (486kmh).
Kelsey Angle, a weather service meteorologist in Kansas City, Missouri, said it was unusual for two such powerful tornadoes to track roughly the same path.
It was the fourth tornado to hit Moore since 1998. A twister also struck in 2003.
Oklahoma City has had more tornado strikes than any other city in the United States," the city government's website said.