A powerful tornado with winds of up to 200 miles (320 kilometres) per hour pulverized an Oklahoma City suburb Monday, hitting at least two schools and wiping out blocks of homes. At least 37 people were reported killed.
The Oklahoma medical examiner's office gave the latest death toll, which was carried by all the major US television networks, which said the number of fatalities was expected to rise.
"Our first responders are stretched," Oklahoma City Mayor Mick Cornett told CNN. "The state, the National Guard are going to be involved."
Reporters for KFOR-TV saw pupils as young as nine being "pulled out" of the school in Moore, a residential community of 55,000 just south of Oklahoma's state capital.
Anxious parents were being kept at a distance while search and rescue workers scrambled to free the pupils.
A second elementary school, Briarwood, was also hit but did not immediately appear to have sustained casualties. Early reports indicated that many students survived.
From its news helicopter, KFOR's cameras captured scenes of widespread destruction, with street after street of single-storey homes in Moore stripped of their roofs and cars piled atop one another.
Utility lines were down and gas lines exposed, triggering localized fires. The Moore Medical Center was evacuated after it sustained damage, a spokeswoman for the hospital told CNN.
The National Guard was called out to help rescue efforts.
Storm spotters estimated the wedge-shaped tornado, which struck in mid-afternoon, to be as big as two miles (3.2 kilometres) wide. It briefly dissipated, only to recycle to the east, threatening the town of Meeker.
"We anticipate that these storms are going to continue to build around Oklahoma," a grim Governor Mary Fallin told CNN, while the National Weather Service urged residents to take cover.
On Twitter, the National Weather Service gave the tornado a preliminary rating of EF-4, indicating that it packed winds of 166 to 200 miles per hour (267-322 km/h) -- more severe than a category five hurricane.
In downtown Oklahoma City, tornado sirens went off at least three times Monday afternoon, and the Interstate 35 highway -- a busy north-south artery through the American heartland -- was closed to all but emergency vehicles.
In Moore, live images from KFOR showed people
wandering among the debris and even a couple of untethered horses from a local
stable that somehow managed to survive the punishing storm. 
"I had no idea it was coming," said a stable worker, who told how he survived the "unbearably loud" twister by taking cover in one of the stalls.
Monday's tornado followed roughly the same west-to-east track as a May 1999 twister that killed 44 people, injured hundreds more and destroyed thousands of homes in Moore and the south of Oklahoma City.
Tornadoes frequently touch down on Oklahoma's wide open plains, but the fact that Monday's twister struck a populated urban area raised fears of a high casualty toll.
Because of the hard ground, few homes are built with basements in which residents can take cover.
Oklahoma City lies well inside the so-called "Tornado Alley", stretching from South Dakota to central Texas that is particularly vulnerable to tornadoes.
On Sunday, a powerful storm system churning through the US Midwest spawned tornadoes in Iowa, Kansas and Oklahoma, destroying homes and killing at least two people, US media reported.
Fallin declared a state of emergency Sunday for 16 Oklahoma counties due to tornados, severe storms and flooding over the weekend.
In Washington, a White House official said President Barack Obama was getting updates "as information come in from the ground" and that the Federal Emergency Management Agency (FEMA) stood ready to provide assistance.
"The administration continues to urge all those in affected or potentially affected areas to follow the direction of state and local officials as this severe weather continues," the official added.
AFP/xq
