Rescuers in Alabama have stepped up the search for survivors after two back-to-back tornadoes ripped across the southern state.

Twenty-three people are confirmed but the death toll is expected to rise further.

"I would describe the damage that we have seen in the area as catastrophic," Lee County Sheriff Jay Jones said.

"It looks almost as if someone took a giant knife and scraped the ground," he said.

"There are slabs where homes formerly stood, there is debris everywhere, trees are snapped, whole ... forested areas are just snapped."

"I cannot recall at least in the last 50 years ... a situation where we have had this loss of life," Jones said.

He said the death toll stood at 23, some of them children. One of the dead was just six years old.

Another young victim, Taylor Thornton, died while visiting a friend in Lee County. "She's gone to heaven. She's only eight years old. It feels not real," her aunt Kay Thornton told NBC affiliate WSFA 12 before bursting into tears.

"We have several people who are still unaccounted for," Jones said. "Unfortunately, we anticipate the number of fatalities may rise as the day goes on."

Other people were hospitalized, some with "very serious injuries."

Search operations for those still missing had to be halted on Sunday night due to hazardous conditions, but were renewed this morning with agencies from across the state and from neighboring Georgia joining the hunt.

The tornado uprooted trees and caused "catastrophic" damage to buildings and roads.

The swath of destruction left by the storm was a half a kilometre wide and stretched for the "several miles that it travelled on the ground," according to Mr Jones.

Still and video images showed trees that had been snapped in two, debris-strewn roads and wrecked houses in the wake of the storm.

More than 6,000 homes were left without power in Alabama, according to PowerOutage.US, while 16,000 suffered outages in neighbouring Georgia.

Television images showed the heavy rain had relented by dusk, but many roads in the worst-hit areas were left littered with debris and impassable.

Residents in the town of Smith Station told local TV news crews of their shock at turning up to work to find their businesses destroyed, and seeing crying co-workers comforting one another.

One bar in the town appeared to have lost its roof and most of its walls, in images screened by MSNBC, while a cell tower was completely destroyed.

"My sister and niece have been under tornado watch and warnings all day in Montgomery ... Prayers up for Alabama," Emmy Award-winning filmmaker Ava Duvernay tweeted.

Last night, US President Donald Trump expressed his condolences to those affected.

"To the great people of Alabama and surrounding areas: Please be careful and safe," he wrote on Twitter.

"Tornadoes and storms were truly violent and more could be coming. To the families and friends of the victims, and to the injured, God bless you all!"

The National Weather Center had issued a tornado warning for areas including Lee County earlier yesterday, calling on residents to: "TAKE COVER NOW! Move to a basement or an interior room on the lowest floor of a sturdy building. Avoid windows."

It tweeted that the southeastern US, struck by a severe storm system, could see "damaging winds, large hail and isolated tornadoes" overnight.

While Mr Jones referred to a single storm, CNN reported that two tornadoes had hit Lee County in quick succession, some of "at least a dozen" it said tore through Alabama and the neighbouring state of Georgia yesterday.

NWS Birmingham suggested that there were multiple twisters in the area, tweeting that the "first tornado to impact Lee County today was at least an EF-3 & at least 1/2 mi wide".

The EF-3 designation - on a scale of 0 to 5 - means the tornado had winds of 218-266km/h.