Shocked Americans are struggling to grasp the magnitude of the worst US tornadoes in almost 40 years, which carved a trail of destruction across the south, claiming at least 295 lives.

The storms were the deadliest since 310 people were killed in 1974 when 148 tornadoes hit several states and authorities have warned that the death toll could rise further.

Entire US towns were destroyed, as six states were struck by huge twisters. One that was captured on extraordinary video footage measured a mile wide – 20 times larger than the typical tornado.

States of emergency were declared by the governors of Alabama, Arkansas, Georgia, Kentucky, Mississippi, Missouri, Oklahoma and Tennessee, and governors called out the National Guard - including 2,000 troops in Alabama - to help with the rescue and clean-up operations.

Alabama, one of America's poorest states, was worst hit. More than 131 people there died, 36 in the city of Tuscaloosa alone. Walter Maddox, its mayor, said the city had been "obliterated".

"I don't know how anyone survived," said Mr Maddox. "It's an amazing scene.

"There are parts of the city I don't recognise and I've lived here my entire life."

James Sykes, a survivor, described watching a "silent monster" suck up the city's streets. "It was full of lightning, devastating everything," Mr Sykes said. Dozens of businesses and emergency service buildings across the city were wiped out.

"We had a major catastrophic event here in Alabama," said Robert Bentley, the state's governor. "We have major destruction in the state." A million people were left without power.

Storms caused the Browns Ferry nuclear power plant near Huntsville, Alabama, to lose power. Officials described the incident as a low-level emergency and said it was under control.

The National Weather Service said it had received 137 tornado reports on Wednesday, bringing to 300 the total number counted in the region since Friday.

In Mississippi, at least 32 people were killed. Another 30 were reported dead in Tennessee, 11 in Arkansas, 13 in Georgia, seven in Virginia, and three in Missouri.

President Barack Obama, who promised swift assistance, said: "Our hearts go out to all those who have been affected by this devastation". He will travel to Alabama to view the damage on Friday.

Mr Obama also paid tribute to the "heroic efforts" of those who were responding to the disaster by clearing up disaster-struck areas and searching for victims.

People throughout the south were left trapped in their homes, colleges and vehicles after fallen trees and flooding left large areas impossible to pass.

The storms appeared to have been the deadliest natural disaster in the US since Hurricane Katrina, which killed more than 1,800 people in Louisiana in 2005.

Further heavy rain and high winds are expected on Saturday, with 21 states throughout the country warned that they could face severe weather.

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