- As it happened: tornado's path of destruction
- Deadliest tornadoes since 1900
- How tornadoes are ranked in severity
- Missing children: 'Our hearts are broken'
Rescue workers are using floodlights to continue the search for students lost at an Oklahoma City primary school demolished by a monster tornado responsible for nearly 100 deaths.
'We thought we were dead': Oklahoma survivor
Donald Trump's effective debate tactics
Clinton and Trump prepare to face off
Election expert: Trump is headed for a win
The Ivory Debate: should the ban be lifted?
Arnold Palmer dies aged 87
Malnutrition grows in war-ravaged Yemen
US, Russia trade blows over Syria
'We thought we were dead': Oklahoma survivor
RAW VIDEO: US man, Ricky Stover, recounts the harrowing experience as the Oklahoma tornado swung the cellar doors open where he sought refuge.
At least 91 people have died, including 20 children, the New York Times reports. Earlier the state medical examiner's office confirmed that 51 people were killed in the tornado.
A further 145 people, including about 70 children, are being treated at local hospitals.
''They are still in the process of picking through the rubble to try to find survivors,'' Jerry Lojka, a spokesman for Oklahoma Emergency Management, said in an interview. ''We know it was on the ground for 40 minutes. It travelled across the entire city of Moore.''
''It's like a war zone down there,'' said Moore resident Melissa Burton, whose children at Kingsgate Elementary School were returned to her unharmed. Her house was destroyed by the tornado, she said.
''My F-150 truck is gone and we don't know where it is,'' Burton said. ''We think it wound up two streets over.''
Measuring about three kilometres wide in places, with winds of up to 321 kilometres per hour, the twister was so violent that it flattened homes, cars, trees and shrubs.
''There are entire blocks that are wiped clean to the foundations,'' Lojka said. ''It went through residential areas, business areas. We know a hospital is heavily damaged, two elementary schools are heavily damaged.''
Moore, a middle class community with a population of 55,000, was worst hit. The tornado arrived in the afternoon as children were preparing to leave school. The Moore Public School district serves more than 20,000 students and is the third largest in the state.
As night descended, rescue workers used floodlights to search for children at the Plaza Towers Elementary School in Moore, Oklahoma, where seven children reportedly drowned, while others were pulled alive from the rubble and passed hand-to-hand by a human chain of rescuers.
Earlier, authorities told local TV news station KFOR that 20 to 30 children could be trapped inside with no survivors expected.
But as the frantic search effort continued, one of the station's reporters said some of the missing children may not have still been at the school when the twister hit.
A second school, Briarwood Elementary, was also devastated by the tornado but local media said the school's parents had accounted for all of their children.
'Just like Twister'
The people of Moore are no strangers to violent twisters, with a 1999 tornado producing the highest winds ever recorded near the Earth’s surface - 486 km/h.
Oklahoma City lies well inside the so-called ‘‘Tornado Alley’’ stretching from South Dakota to central Texas that is particularly vulnerable to tornadoes.
Lando Hite, shirtless and spattered in mud, told KFOR about the storm hitting the Orr Family Farm in Moore, which had about 80 horses.
"It was just like the movie Twister," he said. "There were horses and stuff flying around everywhere."
National Guard troops and volunteers searched for survivors as the sun went down, with the neighbourhoods evacuated and the victims being guided towards overnight shelters. The National Guard will use night vision and infra-red cameras to look for survivors among the rubble.
James Rushing, who lives across the street, heard reports of the approaching tornado and ran to the school, where his five-year-old foster son, Aiden, attends classes. Mr Rushing believed he would be safer there.
"About two minutes after I got there, the school started coming apart," he said.
The students were put in the restroom.
After the tornado roared through, Tiffany Thronesberry said she got a panicked call from her mother, Barbara Jarrell.
"I got a phone call from her screaming, 'Help, help! I can't breathe. My house is on top of me!"' Ms Thronesberry said.
She hurried to her mother's house, where first responders had already pulled her out with cuts and bruises.
Downed power lines and open gas lines posed a risk on Monday night, police Captain Dexter Nelson said.
Obama declares disaster, orders financial support
Oklahoma Governor Mary Fallin told reporters "hearts are broken" for parents looking for their children.
Less than one per cent of all tornadoes reach the wind speed of this storm, which ripped through scores of buildings in a region of the US known as Tornado Alley. US President Barack Obama called Ms Fallin to express his concern.
President Obama declared a major disaster in Oklahoma and ordered the availability of federal financial aid.
The president's declaration makes US government funding available to people affected by the storm in five Oklahoma counties, according to a White House statement issued tonight. The assistance includes grants to cover home repair and temporary housing costs, along with loans to help pay for uninsured property losses, the White House said.
The US Federal Emergency Management Agency continues to conduct damage assessments in the affected areas, and additional assistance may be made available depending on the results, the White House said. Funding will also be available to the state and eligible local governments in the area.
Tornado ranked an EF-4
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.
The tornado formed in less than an hour gave people in the Oklahoma City suburb little time to prepare. The deadly twister darkened as it cut a 32 kilometre path, sucking in debris and earth from the path of destruction below.
In video footage, the dark funnel cloud moved slowly across the landscape, scattering shards of wood, pieces of insulation, shingles and glass over the streets.
The focus quickly turned to Plaza Towers Elementary School, where the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.
Yet the deadly tornado so far ranks below the most deadly in US history, the Tri-state tornado of 1925. That storm, which swept through Missouri, Illinois and Indiana, killed 695 people.
"Oklahoma City has had more tornado strikes than any other city in the United States," the city government's website said.
Chris Zappone with Megan Levy, Nick O'Malley and AP, Bloomberg