MOORE (Oklahoma) (Reuters) - Emergency workers searched for survivors in the rubble of homes, schools and a hospital in an Oklahoma town hit by a powerful tornado, but officials on Tuesday sharply lowered the number of deaths caused by the storm.

The Oklahoma state medical examiner's office said 24 bodies had been recovered from the wreckage of Monday's storm, down from the 51 they had reported earlier. The earlier number likely reflected some double-counted deaths, said Ms Amy Elliott, chief administrative officer for the medical examiner.

The 3km-wide tornado tore through Moore, outside Oklahoma City, on Monday afternoon, trapping victims beneath the rubble. One elementary school took a direct hit and another was destroyed.

Thunderstorms and lightning slowed the rescue effort on Tuesday, but officials lowered the number of bodies recovered.

"We have got good news. The number right now is 24," Ms Elliot said. "There was a lot of chaos."

She said additional bodies could yet be recovered.

Firefighters from more than a dozen fire departments worked all night under bright spotlights trying to find survivors at Plaza Towers Elementary School, which took a direct hit.

Rescuers were sent from other states to join the search.

United States President Barack Obama declared a major disaster area in Oklahoma, ordering federal aid to supplement state and local efforts in Moore after the deadliest US tornado since 161 people were killed in Joplin, Missouri, two years ago.

The White House said Mr Obama would make a statement on the Oklahoma tornado at 10am Eastern Daylight Time (10pm Singapore time).

"The whole city looks like a debris field," Moore Mayor Glenn Lewis told NBC.

"It looks like we have lost our hospital. I drove by there a while ago and it's pretty much destroyed," he said.

There was an outpouring of grief on Plaza Towers' Facebook page, with messages from around the country, including one pleading simply: "Please find those little children."

The National Weather Service assigned the twister a preliminary ranking of EF4 on the Enhanced Fujita Scale, the second-most powerful category of tornado with winds up to 320kmh.

US Representative Tom Cole, who lives in Moore, said Plaza Towers was the most secure and structurally strong building in the area.

"And so people did the right thing, but if you're in front of an F4 or an F5 there is no good thing to do if you're above ground. It's just tragic," he said on MSNBC TV.

At least 60 of the 240 people injured were children, hospital officials said.

Witnesses said Monday's tornado appeared more fierce than the giant twister that was among the dozens that tore up the area on May 3, 1999, killing more than 40 people and destroying thousands of homes. That tornado ranked as an EF5 tornado, with wind speeds of more than 320kmh.

The 1999 tornado ranks as the third-costliest tornado in US history, having caused more than US$1 billion in damage at the time, or more than US$1.3 billion (S$1.6 billion) in today's dollars. Only the devastating Joplin and Tuscaloosa tornadoes in 2011 were more costly.

Mr Jeff Alger, 34, who works in the Kansas oil fields, said his wife Sophia took their children out of school when she heard a tornado was coming, and then fled Moore and watched it flatten the town from afar.

"They didn't even have time to grab their shoes," said Mr Alger, who has five children aged four to 11. The storm tore part of the roof off of his home. He was with his wife at Norman Regional Hospital to have glass and other debris removed from his wife's bare feet.

Moore was devastated with debris everywhere, street signs gone, lights out, houses destroyed and vehicles tossed about as if they were toys.

The dangerous storm system threatened several southern Plains states with more twisters.

Speaking outside Norman Regional Hospital, Ms Ninia Lay, 48, said she huddled in a closet through two storm alerts and the tornado hit on the third.

"I was hiding in the closet and I heard something like a train coming," she said under skies still flashing with lightning. The house was flattened and Ms Lay was buried in the rubble for two hours until her husband Kevin, 50, and rescuers dug her out.

"I thank God for my cellphone, I called my husband for help." Her 7-year-old daughter Catherine, a first-grader at Plaza Towers, took shelter with classmates and teachers in a bathroom when the tornado hit and destroyed the school. She escaped with scrapes and cuts.

At Southmoore High School in Moore, about 15 students were in a field house when the tornado hit. Coaches sent them to an interior locker room and made them put on football helmets, the Oklahoman newspaper said. It said the students survived.

The National Weather Service Storm Prediction Centre warned the town 16 minutes before the tornado touched down at 3:01pm, which is greater than the average eight to 10 minutes of warning, said Ms Keli Pirtle, a spokesman for the centre in Norman, Oklahoma.

The notice was upgraded to emergency warning with "heightened language" at 2.56pm, or five minutes before the tornado touched down, Ms Pirtle said.

The US Federal Aviation Administration imposed a temporary flight restriction that allowed only relief aircraft in the area, saying it was at the request of the police, who needed quiet to search for buried survivors.

Oklahoma activated the National Guard, and the US Federal Emergency Management Agency activated teams to support recovery operations and coordinate responses for multiple agencies.

Briarwood Elementary School, which also stood in the storm's path, was all but destroyed. On the first floor, sections of walls had been peeled away, giving clear views into the building; while in other areas, cars hurled by the storm winds were lodged in the walls.