MOORE, Okla., May 20 (Reuters) - A massive tornado with winds of up to 200 miles per hour (320 kph) devastated the Oklahoma City suburb of Moore on Monday, killing at least 37 people as it tore up entire tracts of homes, two schools and a hospital, leaving a wake of tangled wreckage.

The death toll of 51 was expected to rise, said Oklahoma officials. Some 20 to 30 school children were missing and feared dead beneath the rubble, KFOR television reported, citing unnamed officials from the scene.

Rescue workers raced against the setting sun to find survivors in Oklahoma as the dangerous storm system threatened as many as 10 U.S. states with more twisters.

Police and fire crews pulled third-graders from the devastated Plaza Towers Elementary school in Moore, a KFOR television reporter said from the scene, and aerial video showed teams sifting through the rubble left behind.

"I have never seen anything like this in my 18 years covering tornadoes here in Oklahoma City. This is without question the most horrific," said Lance West, a reporter for KFOR.

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.

'A DEBRIS FIELD'

Television images showed blocks of homes leveled by the powerful tornado, cars piled atop one another and buildings on fire.

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.

While the school was a wreck, nearby playground equipment stood undamaged, though littered with rubble.

Across the street, people picked through the remains of their homes, looking for any possessions they might salvage.

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 (320 kph).

At least 45 people were injured, according to officials of four hospitals.

"They (injured) are coming in minute by minute," said Integris Southwest Medical Center spokeswoman Brooke Cayot. Of the 19 injured there, seven were in critical condition, seven serious and five listed as fair or good, Cayot said.

The University of Oklahoma Medical Center had received at least 20 injured. St. Anthony Healthplex South in Oklahoma City said it received four patients and Midwest Regional received four.

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.

Fire, rescue and emergency medical teams from across the state converged on Moore, and members of the National Guard, activated by Governor Mary Fallin, were on the scene, said Terri Watkins, a spokeswoman for the Oklahoma Department of Emergency Management.