(Reuters) - Pre-dawn emergency workers searched feverishly for survivors in the rubble of homes, primary schools and an hospital in an Oklahoma City suburb ravaged by a massive Monday afternoon tornado feared to have killed up to 91 people and injured well over 200 residents.
The 2-mile(3-km) wide tornado tore through town of Moore outside Oklahoma City, trapping victims beneath the rubble as one elementary school took a direct hit and another was destroyed.
Reporters were cleared back from Plaza Towers Elementary School, which sustained a direct hit Oklahoma Lieutenant Governor Todd Lamb told CNN. But television pictures showed firefighters from more than a dozen fire departments working under bright spotlights to find survivors.
President Barack Obama declared a major disaster area in Oklahoma, ordering federal aid to supplement state and local efforts in Moore after the deadliest U.S. tornado since one killed 161 people in Joplin, Missouri, two years ago.
The White House said Obama would make a statement on the Oklahoma tornado at 10 a.m. EDT (1400 GMT). It said the president was brieved overnight on the tornado tragedy.
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."
A separate Facebook page set up to reunite people in the area hit by a tornado on Sunday with their belongings and pets also showed entries for Moore residents overnight.
Another elementary school, homes and a hospital were among the buildings leveled in Moore, leaving residents of the town of about 50,000 people stunned at the devastation and loss of life. Many residents were left without power and water.
The Oklahoma medical examiner said 20 of the 91 expected to have been killed were children. The office had already confirmed 51 dead and had been told during the night by emergency services to expect 40 more bodies found in the debris, but had not yet received them.
At least 60 of the 240 people injured were children, area hospitals said.
The National Weather Service assigned the twister a preliminary ranking of EF4 on the Enhanced Fujita Scale, meaning the second most powerful category of tornado with winds up to 200 mph.
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, meaning it had winds over 200 mph.
The 1999 event in Oklahoma ranks as the third-costliest tornado in U.S. history, having caused more than $1 billion in damage at the time, or more than $1.3 billion in today's dollars. Only the devastating Joplin and Tuscaloosa tornadoes in 2011 were more costly.
"I am close to the families of all who died in the Oklahoma tornado, especially those who lost young children. Join me in praying for them," Pope Francis said in a Twitter message.
Jeff Alger, 34, who works in the Kansas oil fields on a fracking crew, 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 a few miles away.
"They didn't even have time to grab their shoes," said 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. The area around Moore faces the risk of severe thunderstorms on Tuesday, which could hamper rescue efforts.
Speaking outside Norman Regional Hospital 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 Lay was buried in the rubble for two hours until her husband Kevin, 50, and rescuers dug her out.
"I thank God for my cell phone, I called me husband for help."
Her daughter Catherine, seven, a first-grader at Plaza Towers Elementary School, took shelter with classmates and teachers in a bathroom when the tornado hit and destroyed the school. She escaped with scrapes and cuts.
The National Weather Service Storm Prediction Center provided the town with a warning 16 minutes before the tornado touched down at 3:01 p.m. (2001 GMT), which is greater than the average eight to 10 minutes of warning, said Keli Pirtle, a spokeswoman for the center in Norman, Oklahoma.
The notice was upgraded to emergency warning with "heightened language" at 2:56 p.m., or five minutes before the tornado touched down, Pirtle said.
Television media measured the tornado at more than 2 miles wide, with images showing entire neighborhoods flattened.
The U.S. Federal Aviation Administration (FAA) imposed a temporary flight restriction that allowed only relief aircraft in the area, saying it was at the request of police who needed quiet to search for buried survivors.
Oklahoma activated the National Guard, and the U.S. 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.
The number of injured as reported by several hospitals rose rapidly throughout the afternoon.
"The whole city looks like a debris field," Glenn Lewis, the mayor of Moore, told NBC.
"It looks like we have lost our hospital. I drove by there a while ago and it's pretty much destroyed," Lewis said.
The massive twister struck at the height of tornado season, and more were forecast. On Sunday, tornadoes killed two people and injured 39 in Oklahoma.
(Additional reporting by Lindsay Morris, Carey Gillam, Nick Carey, Brendan O'Brien and Greg McCune; Writing by Daniel Trotta and Nick Carey; Editing by Alison Williams and W Simon)