At least 24 people are confirmed dead, including seven children, and many are missing after a tornado tore through Oklahoma City, US officials say.
Worst hit was Moore, south of the city, where neighbourhoods were flattened and schools destroyed by winds of up to 200mph (320km/h).
About 120 people are being treated in hospitals. Earlier reports said more than 90 people had died.
President Barack Obama has declared a major disaster in Oklahoma.
He also ordered federal authorities to join in the search efforts which have continued throughout the night.
Monday's twister hit Moore, a suburb of about 55,000 people, at 15:01 (20:01 GMT) and remained on the ground for about 45 minutes.
On Tuesday morning, the chief administrative officer of the Oklahoma City Medical Examiner's Office told reporters that the death toll had been revised down.
"Fifty-one deaths were reported to the OCME during the early phases of the recovery efforts yesterday," Amy Elliott said.
"To date, 24 deceased victims of the tornado have been transported to our Oklahoma City Office, and positive identification have been made in the vast majority of those, and these are ready for return to their loved ones."
Earlier, local authorities said a further 40 bodies had been found but not identified.
Several children are believed to have been killed when Plaza Towers Elementary school took a direct hit. The storm tore off the building's roof and knocked down walls.
"The school was flattened. The walls were pancaked in," Oklahoma's Lieutenant Governor Todd Lamb told the BBC.
"There's still roughly two dozen children that are missing. There have been some bodies recovered from that school and it's absolutely horrific and devastating."
Another school - Briarwood Elementary - was also damaged, and teachers were later seen leading pupils out to safety.
Lt Gov Lamb said Moore had been prepared for a tornado, but not of this strength: "There was a warning in place, but when it veers and is as extensive as this one is - two miles wide - on the ground for 45 minutes, that is just wreaking havoc that it's hard to get out of the way of."
Oklahoma Governor Mary Fallin said it was a "tragic" day.
More than 200 Oklahoma National Guardsmen as well as out-of-state personnel have been called in to assist the search-and-rescue effort.
The storm destroyed several areas, leaving a tangle of ruined buildings, piles of broken wood, overturned and crushed cars, and fires in some places.
The BBC's Alastair Leithead in Oklahoma says many tornadoes in the region hit the open plain, but this one struck a residential area.
Many houses are built on hard ground without basements, so residents did not have recourse to shelter, our correspondent adds.
'School coming apart'
"We locked the cellar door once we saw it coming, it got louder and next thing you know is you see the latch coming undone," survivor Ricky Stover said.
"We couldn't reach for it and it ripped open the door and just glass and debris started slamming on us and we thought we were dead, to be honest."
Melissa Newton, another survivor, said: "There's shingles and pieces of sheet rock and wood in our yard and all across our neighbourhood. Some homes are completely gone. It's devastating."
James Rushing said he had hurried to Plaza Towers Elementary School, where his foster son Aiden was a pupil, to see it destroyed by the storm.
"About two minutes after I got there, the school started coming apart," he told the Associated Press news agency.
The National Weather Service (NWS) said Monday's tornado had generated winds of up to 200mph.
"It's certainly the most powerful tornado that I've ever dealt with in my 20 years with the weather service," NWS meteorologist Rick Smith in Norman, Oklahoma, told the BBC.
The NWS said the tornado measured EF-4 on the five-point Fujita scale - the second most powerful type.
The town of Moore was hit by a severe tornado in May 1999, which had the highest winds ever recorded on Earth.
But Betsy Randolph of the Oklahoma Highway Patrol told local news station Skynews 9 that the damage on Monday appeared to exceed that of the 1999 tornado.
Tornadoes, hail and high winds also hit Iowa and Kansas, part of a storm system stretching from Texas to Minnesota.
On Sunday, a tornado smashed a trailer park on Highway 102 near Shawnee, Oklahoma.
Oklahoma's state medical examiner confirmed earlier on Monday that two people had been killed in the area.
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