51 killed as powerful tornado slams Oklahoma City

At least 51 people, including seven children at an elementary school, were killed as a massive tornado slammed Oklahoma City in south central US leaving widespread destruction in its wake.

Emergency personnel were scouring the rubble at flattened Plaza Towers Elementary School in Moore, Oklahoma, hours after the tornado struck Monday afternoon, a video from CNN affiliate KFOR showed.

As nightfall approached, determined searchers in hard hats dug in the debris for students possibly trapped, but officials cited by the news channel described the work as a recovery, not rescue, effort.

Footage from local television stations also showed a number of other levelled buildings and a funnel cloud stretching from the sky to the ground, kicking up debris.

The tornado was estimated to be at least 2 miles wide at one point as it moved through Moore, in the southern part of the Oklahoma City metropolitan area, KFOR reported.

Storm damage has been reported in Cleveland County, which includes Moore; McClain County, which includes Newcastle; and Oklahoma County, CNN reported citing Oklahoma Department of Emergency Management representative Terri Watkins.

"After the ear-shattering howl of the killer storm subsided, survivors emerged from shelters to see an apocalyptic vision -- the remnants of cars twisted and piled on each other to make what had been a parking lot look like a junk yard," according to a CNN report.

"Bright orange flames roaring from a structure that was blazing even as rain continued to fall," it said.

The preliminary rating of damage created by the tornado is at least EF4 (winds 166 to 200 mph) -- the second most severe classification on a scale of zero to five -- the National Weather Service said.

Even as authorities and rescue workers struggled to get handle on the damage, National Oceanic and Atmospheric Administration's Storm Prediction Centre warned the worst may be yet to come.

"These storms are going to continue producing additional tornadoes. They'll also produce some very, very large hail, perhaps larger than the size of baseballs," NOAA's Bill Bunting told CNN.

The severe weather came after tornadoes and powerful storms ripped through Oklahoma and the Midwest earlier Monday and on Sunday damaging or destroying an estimated 300 homes.