The number of people confirmed dead after one of the US' strongest tornadoes ever has been revised to 24, amid a renewed hunt for survivors.
At least seven of those killed by ferocious winds of up to 200mph in the southern suburb of Moore were children, with one elementary school being directly hit.
Entire neighbourhoods were flattened, rendering thousands of people homeless and leaving buildings on fire.
Officials had previously said 51 people were confirmed dead, but later revised that figure downwards to 24.
Amy Elliott, chief administrative officer at the Oklahoma City Medical Examiner's Office, said: "We have got good news. The number right now is 24. There was a lot of chaos."
The earlier figure of 51 dead may have included some double-reported casualties, she added
She previously said her office had been told by emergency services to expect another 40 bodies that had been retrieved from the rubble.
And as rescue workers continued to search for survivors, forecasters warned of more severe weather in region.
Weather Channel meteorologist Kevin Roth said the east and south of Oklahoma City were the areas most at risk, with other parts of 'tornado alley' also threatened.
"Tornadoes, damaging wind gusts and large hail are possible throughout the threat area," he said.
"Another day of large and devastating tornadoes is possible this time from central/east Texas into central Arkansas."
Oklahoma Highway Patrol trooper Betsy Randolph said 101 people who were trapped alive had been found and rescued.
Several children were pulled alive from the wreckage of Plaza Towers Elementary School after the devastating tornado - between one and two miles wide - reduced the building to heaps of rubble and twisted metal.More pictures as tornado devastates Oklahoma:
Roughly 500 students attend the school. It is unclear whether any had been evacuated before the twister hit, but local media reported some children were taken to a nearby church.
Firefighters were at the scene digging through the debris to reach any pupils possibly trapped inside.
A second elementary school, Briarwood, was also hit but did not appear to have sustained casualties, while Moore's hospital complex was also badly damaged.
Emergency crews pledged to work through the night across the city to look for survivors, amid reports that another, smaller, storm was on its way to the town of 55,000 people.
Officials at two hospitals said they were treating more than 140 patients, including around 70 children. Dozens of people were said to be in a critical condition.
President Barack Obama - who was due to deliver a statement at 3pm UK time (11am EDT) - spoke to Oklahoma Governor Mary Fallin on the phone and promised the state all the help it needs as he declared the devastating tornado a major disaster.
Ms Fallin told a news conference "hearts are broken" for the parents looking for their children.
"This is a very sad day for the state of Oklahoma - a very hard day and very tragic day," she said.
"Our prayers and our thoughts are with all the Oklahoma families that have been hit hard by this terrible storm. We are doing every single thing that we can to assist those that are in need right now."
She said communication was hard with power lines and mobile phone towers hit by the storm.
The governor said she had deployed 80 Oklahoma National Guard members to help with the recovery effort and assist search and rescue teams who she said were "looking under every single piece of debris" to find anyone that might be injured or lost.
She had already declared a state of emergency for 16 Oklahoma counties due to the tornado threat on Sunday, and added five more on Monday after the storms hit the state capital.
Eric Olsen, a radio journalist, said parts of Moore resembled a war zone.
He told Sky News: "The devastation here is absolutely mind-blowing. It looks like a bomb has absolutely levelled it. It looks like something you would see in maybe Afghanistan or Iraq."
The National Weather Service gave the twister a preliminary EF-4 classification - on a five-point scale - with winds up to 200mph.
Weather service meteorologist Kelsey Angle said fewer than 1% of all tornadoes ever reach EF-4 or EF-5 levels.
Several other tornado warnings were also in effect following the devastating twister.It came just a day after two people were confirmed killed by a tornado nearby .
Residents of Moore had been urged to take shelter as the broad, dark funnel cloud moved through the area for around 35 minutes before dissipating.
KFOR-TV's news helicopter showed huge swathes of buildings and homes completely levelled, with nothing but wreckage left. Some homes were taken down to their concrete slabs.
Sky News US Correspondent Dominic Waghorn described the damage, saying "whole neighbourhoods just wiped off the map, homes literally stripped to their foundations".
Aerial footage showed vehicles crumpled and overturned in piles of debris on the motorway, and buildings that had become unrecognisable jumbles of rubble.
Utility workers rushed to shut off electricity and gas to the area to prevent further danger from live power lines or natural gas leaks.Footage of the storm showed the monster twister slowly moving through the area and the flashes of power lines blowing.
In Tulsa - 100 miles away - residents reported the debris from the Moore tornado raining down on their neighbourhoods.
The huge tornado was the most recent in a series of twisters that have ravaged towns in the midwest US in recent days, with a line of violent storms that have stretched from the Canadian border down into Texas.
The stretch of the country is known as "tornado alley" and residents are trained in how to take shelter. Most towns and cities are equipped with storm sirens that can warn of a coming tornado half an hour before it hits.
The same suburb was hit hard by a tornado in 1999, when the storm produced the highest winds ever recorded near the Earth's surface - of up to 302mph.
The Queen said she was "deeply saddened" by the loss of life and devastation caused by the tornado and sent her "deepest sympathies" to all those affected.
Country music star Toby Keith, who grew up in the town, said it would persevere.
"Hometown got hit for the gazillionth time. Rise again Moore Oklahoma," he tweeted.