Devastating tornadoes too numerous to count roared across the American states of Oklahoma and Kansas late yesterday . At least 46 people were killed and hundreds were left injured and homeless.

A tornado is an extremely violent windstorm that is usually created within a thunderstorm or hurricane. The recognizable shape of the tornado is the swirling funnel cloud that extends from the base of a storm cloud to the ground.

The Oklahoma state medical examiner said 40 people were killed in Oklahoma alone in Monday's storms, the largest of which formed about 110 kilometres southwest of Oklahoma City - cutting a path at least a kilometre wide as it moved north and east.

About 400 kilometres miles north, a tornado spawned by the same storm system tossed mobile homes like tin cans, damaged houses and killed at least five people in Wichita, Kansas and its suburb of Haysville. Hospitals reported treating more than 80 people.

Fred Irvin of the Sedgwick County Emergency Preparedness Office said today that the initial count of 10 deaths proved wrong when authorities realized victims had been counted twice. But authorities still fear more dead will be found.

"It is worse than what you can see," says Bob Thompson, a battalion chief for the Sedgwick County Fire Department. "We'll probably find more deaths."

The storm system began in southwest Oklahoma and moved into the neighboring state of Kansas. It included several tornadoes, one of which was nearly two kilometres wide at times.

Witnesses say the twister was like a huge white train that came crashing through their homes.

Chad Harris was with seven people inside his mobile home in Haysville when it was flipped over and demolished. Two of his companions were critically injured.

"I have no trailer," Harris said. "We all rolled in it. It was the worst experience in my life."

The Oklahoma City twister is ranked as an F-4 tornado, the second-strongest classification.

An F-4 tornado has a wind speed of 333-419 kilometres per hour. It can level well-constructed houses, structures with weak foundations can be blown off some distance, cars are thrown and objects like trees can become airborne like missiles.

Scientists rank tornadoes on the Fujita scale of intensity, named after a University of Chicago researcher. On average, only one F-5 tornado hits the United States each year.

1. When did devastating tornadoes too numerous to count roar across the American states of Oklahoma and Kansas?