MOORE, Okla., May 20 (Reuters) - A 2-mile-wide (3-km-wide) tornado tore through the Oklahoma City suburb of Moore on Monday, killing at least 51 people while destroying entire tracts of homes, piling cars atop one another, and trapping two dozen school children beneath rubble.
Twenty of the 51 confirmed deaths were children, the Oklahoma medical examiner said, and at least 45 of the 230 people injured were children, according to area hospitals. It was the deadliest U.S. tornado since one killed 161 people in Joplin, Missouri, two years ago.
President Barack Obama declared a major disaster area in Oklahoma, ordering federal aid to supplement state and local efforts.
Rescue teams raced against the setting sun and worked into the darkness in search of survivors throughout the wide swath of devastation, while the dangerous storm system threatened several southern Plains states with more twisters. Severe weather was expected through the night from the Great Lakes south to Texas.
Emergency crews searched the rubble of Plaza Towers Elementary School for two dozen missing children, Oklahoma Lieutenant Governor Todd Lamb said. The school took a direct hit from the tornado, Lamb told CNN.
The town of Moore, population about 50,000, was devastated with debris everywhere, street signs gone, lights out and houses completely obliterated.
Another elementary school and a hospital were among the buildings leveled.
"We thought we died because we were inside the cellar door...It ripped open the door and just glass and debris started slamming on us and we thought we were dead to be honest," survivor Ricky Stover said while surveying the devastated remains of his home.
Cyndi Christopher was at work and went to pick up her son from daycare when she heard the storm warning. After taking her son home, she was forced to flee when she noticed the storm was coming their way.
"I drove as fast as I could and I outran the storm," Christopher 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 miles per hour (320 km per hour).
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.
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. local time (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 local police who wanted 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.
SCHOOL IN TWISTER'S PATH
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, affording clear views into the building, while in other areas, cars hurled by the storm winds were lodged in the walls.
Across the street, people picked through the remains of their homes.
The number of injured as reported by several hospitals rose rapidly throughout the afternoon.
Oklahoma University Medical Center alone was treating 65 patients, 45 of them children, though it was no longer expecting a further mass influx of casualties, spokesman Scott Coppenbarger said.
Moore Medical Center sustained significant damage.
"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; Editing by Jim Loney and Lisa Shumaker)