Media playback is unsupported on your device Media caption The BBC's Alastair Leithead reports on the landscape left behind

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.

At the scene There is lightning in the sky above Moore, flickering on and off every 10 or 20 seconds; at ground level the only light is from the blue and red flashing lights of emergency vehicles and the bright portable lights with generators at their bases. It is deceptively calm; insects chirrup in the background. Everywhere is covered in a light spray of soggy debris and a film of mud. At the deserted Moore police station, cars are spattered with mud. Larger concrete structures are still standing - a cinema, a supermarket, reminders of what there was before the tornado struck. But houses, offices and shops have been torn apart by the force of the winds that ripped through here, sturdy telephone poles snapped at their bases, street signs sent flying. And most disconcerting - though unsurprising - is the complete absence of any residents. In pictures: Tornado before and after

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.

Image caption The huge tornado ripped through a suburb of Oklahoma City, flattening entire neighbourhoods. Image caption The tornado - said to be at least a mile-wide (1.6km) - destroyed a school in Moore, just south of the city. Image caption Rescuers pulled a number of children alive from the wreckage of Plaza Towers Elementary School. Image caption But across the city at least 20 children have been killed and others could be trapped. Image caption Some pupils were carried away by their distraught parents and others. Image caption President Barack Obama ordered federal authorities to join in the search efforts which continued throughout the night. Image caption It was at least the third tornado to strike Moore since 1999, when more than 40 people were killed. Image caption The Moore Medical Center was destroyed by the tornado. Image caption Rescue workers helped free at least 15 people trapped in the debris from the building. Image caption Meteorologists said the twister generated winds of up to 200mph (321km/h) which reduced some districts to rubble. Image caption Teachers evacuated pupils from another school - Briarwood Elementary - which was also hit by the twister. Image caption Residents returned to their homes in Moore, a suburb of about 55,000 people, to assess the damage. Image caption Here Dana Ulepich searches inside a room left standing at the back of her house. Image caption Resident Rachel Hilton holds stray kittens she found in the debris of her parent's home. previous slide next slide

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."

Media playback is unsupported on your device Media caption The damage wrought by the tornado was "unlike anything that I've ever seen before," said Rick Smith of the National Weather Service

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.

Worst US tornadoes since 1900 18 May 1902 114 deaths in Goliad, Texas

114 deaths in Goliad, Texas 24 April 1908 143 deaths in Amite, Louisiana and Purvis Mississippi

143 deaths in Amite, Louisiana and Purvis Mississippi 18 March 1925 695 deaths in Missouri, Illinois and Indiana

695 deaths in Missouri, Illinois and Indiana 5 April 1936 216 deaths in Tupelo, Mississippi

216 deaths in Tupelo, Mississippi 6 April 1936 203 deaths in Gainesville, Georgia

203 deaths in Gainesville, Georgia 9 April 1947 181 deaths in Woodward, Oklahoma

181 deaths in Woodward, Oklahoma 22 May 2011 158 deaths in Joplin, Missouri. Source: National Oceanic and Atmospheric Administration

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.

Media playback is unsupported on your device Media caption Ricky Stover: "We thought we were dead"

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.