Crews have searched for more victims of deadly tornadoes that killed more than 50 people and injured hundreds more as they tore across four US states, ripping off a shopping mall roof, demolishing mobile homes and blowing apart warehouses.

More than 50 tornadoes touched down as a series of rare winter thunderstorms rolled through five states late on Tuesday and early on Thursday, killing at least 54 people.

The victims included 28 people in Tennessee, 13 in Arkansas, seven in Kentucky and four in Alabama, emergency officials said.

The twisters, which also slammed Mississippi, were part of a rare spasm of winter weather that raged across the US's midsection at the end of the Super Tuesday primaries in several states.

Two states hit by the tornadoes, Arkansas and Tennessee, were among the 24 states that held nominating contests before November's presidential election.

As the extent of the damage quickly became clear, candidates including Hillary Clinton, Barack Obama and Mike Huckabee paused in their victory speeches to remember the victims.

President George W Bush said he called the governors of Alabama, Arkansas, Kentucky, Mississippi and Tennessee and assured them the administration was ready to help and to deal with any emergency requests.

"Loss of life, loss of property - prayers can help and so can the government," Bush said.

"I do want the people in those states to know the American people are standing with them."

Before dawn on Wednesday, the system moved on to Alabama, bringing heavy rains and gusty winds, causing several injuries in counties northwest of Birmingham.

An apparent tornado damaged eight homes in Walker County.

North-east of Nashville, a spectacular fire visible for miles erupted at the Columbia Gulf Natural Gas pumping station northeast of Nashville, sending flames shooting 120-150 metres in the air, said Tennessee Emergency Management spokesman Donnie Smith.

No one was killed in the blast, said Brent Archer, a spokesman for Houston-based Nisource Gas Transmission, adding that the plant apparently took a direct hit from the tornado.

In Memphis, high winds collapsed the roof of a Sears department store at a mall. Debris that included bricks and air conditioning units was scattered on the parking lot, where about two dozen vehicles were damaged.

In Mississippi, Desoto County Sheriff's Department Cmdr Steve Atkinson said a twister shredded warehouses in an industrial park in the city of Southaven, just south of Memphis. He likened the destruction to a bomb going off.

Winter tornadoes are not uncommon. The peak tornado season is late winter through midsummer, but the storms can happen at any time of the year with the right conditions.

But this batch was the nation's worst in a 24-hour period since May 3, 1999, when some 50 people died in Oklahoma and Kansas.

And the death toll ranks among the top 15 from tornado outbreaks since 1950, said Greg Carbin, the warning coordination meteorologist at the centre in Norman, Oklahoma, just south of Oklahoma City.

The tornadoes could be due to La Nina, the cooling of the tropical Pacific Ocean that can cause changes in weather patterns around the world. It is the opposite of the better-known El Nino, a periodic warming of the same region.