Rescuers searched for survivors Tuesday after a massive tornado tore through a suburb in the US state of Oklahoma, destroying two schools and killing up to 91 people, including 20 children.

US President Barack Obama declared a "major disaster" as rescuers combed through smashed homes and the collapsed remains of an elementary school in Moore, where even residents with long memories of past twisters were shocked by the devastation.

The storm flattened block after block of homes, setting off fires, downing power lines and tossing cars across a miles-wide swath of destruction that recalled some of the worst US natural disasters of the last decade.

Stunned weather forecasters described an epic two-mile (three-kilometer) wide storm, as news helicopter footage showed a dark funnel plowing through densely packed suburbs near Oklahoma City, capital of the Midwestern state.

"We've had a massive tornado, a huge one," Oklahoma Governor Mary Fallin told a news conference shortly after the mid-afternoon storm, which struck near the end of the school day Monday.

The dead included at least 20 children, most of them under the age of 12, Amy Elliott of the state medical examiner's office told AFP.

She later said she could not confirm a rise from an earlier official toll of 51 but that she had been told to prepare for 40 more bodies.

CNN reported that at least 145 people had been hospitalized.

Reporters for local broadcaster KFOR-TV on Monday saw children as young as nine being pulled out of the Plaza Towers Elementary 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 students.

A second elementary school, Briarwood, was also hit but did not appear to have suffered casualties.

The Moore Medical Center was evacuated after it sustained damage, and state authorities called out the National Guard to help rescue efforts as Obama ordered federal aid to supplement local recovery efforts.

The president was to be briefed on the disaster by top officials before delivering a statement at the White House at 10:00 am (1400 GMT).

Rescue operations already hindered by the mounds of debris and fallen power lines could encounter further obstacles as the forecast calls for another bout of severe weather in the region.

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 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 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 track as a May 1999 twister that killed 44 people, injured hundreds more and destroyed thousands of homes.

Tornadoes frequently touch down on Oklahoma's wide open plains, but Monday's twister struck a populated urban area and raised fears of a high casualty toll.

Because of the hard ground, few homes are built with basements or storm shelters in which residents can take cover.

Oklahoma City lies inside the so-called "Tornado Alley" stretching from South Dakota to central Texas, an area particularly vulnerable to tornadoes.

But Moore's residents were shocked at the sprawling moon-like landscape left behind by the massive twister.

"There's nothing left of my house," an unidentified woman told CNN.

"The front is still standing but the back is gone. My bathroom honestly is untouched. We've lost animals. We've lost everything," she said.

Some 35,000 people remained without power early Tuesday, according to OG&E, the local utility.

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.