US authorities say the number of people killed in a series of tornadoes and thunderstorms across the southern United States has risen to at least 292, making it the country's deadliest tornado outbreak in almost four decades.

Alabama Governor Robert Bentley says the tornadoes that struck his state on Wednesday killed at least 194 people, by far the highest toll of the eight southern states hit by deadly storms. Speaking Thursday, he said Alabama's final death toll may not be known for another day or two.

US President Barack Obama said the loss of life has been "heartbreaking, especially in Alabama." In a White House address, he said the damage from the storms has been "catastrophic" and promised the federal government will do "everything it can" to help the region recover.

"It looks like we got hit by 5,000 pound bomb here."

The National Weather Service says about 150 tornados tore through parts of the southern states of Mississippi and Alabama on Wednesday. Alabama's city of Tuscaloosa was one of the hardest hit. Several buildings were flattened, and many city operations are unable to function.

Water dripped through the roof of the grocery store Mike Honeysutt manages in Tuscaloosa. He watched as it was destroyed.

"The power went out and the building started shaking, the windows were shaking and part of the roof was flying off the building, the windows came into the front and then the wind started blowing stuff of the shelves," said Honeysutt.

Honeysutt said the tornado was gone 15 or 20 seconds later. The damage was not and many people are missing among the rubble.

Alabama Governor Robert Bentley has deployed 2,000 National Guard troops to assist in search-and-rescue efforts.

"We do have major destruction in the state, especially in the northern part of Alabama," said Bentley. "We have 131 confirmed fatalities at the present time. We expect that number to rise today. In fact, we are sure it will. There may be as many as a half million to a million people in the state without power."

Bentley also reported the Browns Ferry nuclear power plant in northern Alabama automatically shut down after it lost power to its three units. The US Nuclear Regulatory Commission said the plant's safety systems are operating as needed.

The Federal Emergency Management Agency has deployed teams to the region to assist in response efforts, after US President Barack Obama declared a state of emergency in Alabama.

Governor Bentley said Alabama was as prepared as it could be to face tornados, with winds averaging 400 to 500 kilometers per hour. That measure is the most destructive on the Fujita Tornado Damage Scale.

"When you have a catastrophic F4 and F5 tornado that hits, there is not much you can do to change the outcome of that," he said. "But we did have a good response from the weather bureau and we have had a good response from our Emergency Management Agency. We were prepared."

FEMA Director Craig Fugate said the violent streak of tornados that have struck the southern United States in recent weeks is not uncommon.

"Actually what we are seeing is springtime. Unfortunately many people think of Oklahoma as tornado ally and forget that the southeast United States actually has a history of actually longer and more powerful tornados that stay on the ground longer," said Fugate.

Severe weather that began Monday also caused dozens of fatalities in Georgia, Tennessee, Virginia and Arkansas, making it the deadliest series of storms to hit the southern US region in four decades.

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（来源：VOA 编辑：崔旭燕）