Health officials said 40 people were killed in Oklahoma alone in Monday's storms, which local people described as the worst in living memory.

The BBC's Tom Carver: "A vast vacuum cleaner uprooting everything in its path"

In what forecasters described as a "super outbreak of tornadoes," windspeeds reached more than 200 mph (300 kmph), and a tornado up to one mile wide struck Oklahoma City.

Some reports say as many as 48 separate tornadoes were recorded across the region.

Local TV reports showed pictures of dark grey funnels tearing through towns and fields, whipping up cars and clouds of debris with bright flashes as the wind snapped power lines and smashed electricity sub-stations.

Hailstones the size of golfballs were reported in the north of Oklahoma state.

Neighbourhoods levelled

Officials say so far most of the confirmed dead come from within the city limits of Oklahoma City where entire neighbourhoods were levelled.

The Governor of Oklahoma, Frank Keating, has declared a state of emergency, saying whole communities had disappeared "after what looked as though a battle had taken place".

Officials there estimate that as many as 1,000 homes may have been destroyed, as winds tossed mobile homes in the air and flattened houses.

"I watched from my house when the tornado crossed Oklahoma City; it was a monster," said Dave Imy, a meteorologist at the National Weather Service's Storm Prediction Centre in nearby Norman.

Also hit hard was Wichita in Kansas, about 160 miles (250 km) north of Oklahoma City, with at least six people reported dead. Several mobile homes in south Wichita were blown into a lake and many other homes were damaged.

In Oklahoma City families survey damage to their homes

Area forecasters said it was possible some areas could have been hit by storm of F-6 intensity, a force considered catastrophic.

Mike Hammer, spokesman for the Oklahoma County Emergency Management Office, said it was too early to say how high the death toll would go.

"There are so many communities hit so badly, it's anybody's guess as to how high the death toll will be," he said.

Seeking cover

Kirsten Mcintyre of Oklahoma City's Channel 25 TV: "It has been a long day here in Oklahoma City"

Residents were reported to have taken cover in closets, basements and under mattresses and local hospitals were swamped with casualties hit by flying debris.

"We're getting hammered with traumas right now," said an official at Wichita's Via Christi St. Francis hospital.

The tornadoes left a trail of destruction

"It's real chaotic," said Shara Findley, spokeswoman for Hillcrest Health Center in Oklahoma City. "We are getting so many injures we are just tagging them and bringing them in. We're getting everything you can think of."

As hundreds of homes were flattened, Oklahoma Governor, Frank Keating, mobilised the National Guard to comb the debris and combat a spate of gas fires.

"The magnitude of this is just unprecedented," he said. "Hundreds of millions of dollars in property damage and literally hundreds and hundreds of homes destroyed."

Tornado conditions

National Weather Service forecaster, David Andra, told reporters that weather conditions in the area had produced ideal conditions for the formation of supercells - the massive rotating thunderstorms that form the basis for the strongest tornadoes.

"It is just the perfect type of atmosphere, very unstable and with wind shear, to create supercells," he said.

The BBC's Edward Stourton explains how this phenomenon occurs

Overnight police and rescue workers combed through the debris searching for survivors but relief efforts were hampered overnight by widespread power cuts.

Officials said they expected to find more casualties when daybreak made searching easier.