A monstrous tornado as much as 1.6 kilometers wide roared through suburbs of Oklahoma City, killing up to 91, flattening entire neighborhoods with winds of up to 320 kilometers per hour.

The official death toll from the tornado that swept through the Oklahoma City suburb of Moore is expected to rise to 91 from 51, the Oklahoma medical examiner's office said early on Tuesday.

The office had not yet received the 40 more bodies that had been retrieved from the rubble in Moore, but had been told by emergency services to expect them, an official said.

At least one school took a direct hit; twenty of the fatalities are children. Officials at two hospitals said they're treating nearly 60 patients, including more than a dozen children. At least 10 people were in critical condition.

The powerful tornado, rated the second highest strength level of EF4 and packing winds of up to 320 kph, touched down at mid-afternoon and devastated a wide area of the town.

In video of the storm, the dark funnel cloud could be seen moving slowly across the landscape, scattering shards of wood, pieces of insulation, shingles and glass over the streets.

At Plaza Towers Elementary School, the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.

Several children were pulled alive from the rubble. Rescue workers passed the survivors down a human chain to a triage center in the parking lot.

Downed power lines and open gas lines posed a risk in the aftermath of the storm, Police Capt. Dexter Nelson said.

The same suburb was hit hard by a tornado in 1999. That storm had the distinction of producing the highest winds ever recorded near the Earth's surface - 486 kph.