A severe storm system, which spun off tornadoes and left scattered destruction, has killed at least 18 people on a two-day sweep across the southern region of the United States.

At least 14 people were killed on Sunday in Georgia, according to officials, as the fast-moving storms tore across the state throughout the day, with at least one deadly tornado reported before dawn and violent storms still rumbling after nightfall.

Four other people were killed on Saturday in the state of Mississippi, reports said.

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The enormous system put millions of people in the southern US on edge during a weekend of violent weather that left crumpled trailer homes, downed trees and other damage in the hardest-hit communities from Mississippi to Georgia.

On Sunday, President Donald Trump pledged federal assistance to the ravaged areas.

"The tornadoes were vicious and powerful and strong and they suffered greatly," he said during an event in the White House.

"So, we'll be helping out."

The severe weather threat was still continuing on Sunday night in some parts, extending into North and South Carolina, as well as north Florida, according to reports.

The day's deadliest toll came before daybreak on Sunday when an apparent tornado blew through a mobile home park in south Georgia, shearing away siding, upending homes and killing seven people.

"There are houses just demolished," Norma Ford, a resident of Albany, Georgia's largest city with some 76,000 people, told The Associated Press news agency.

Nathan Deal, Georgia's governor declared a state of emergency in seven southern Georgia counties, freeing up state resources to assist with recovery efforts.

January tornado outbreaks are rare, but not unprecedented, in the US, particularly in southern states.

Data from the Storm Prediction Center shows that, over the past decade, the nation has seen an average 38 tornadoes in January, ranging from a high of 84 in 2008 to just four in 2014.

The last time the prediction centre issued a high-risk weather outlook - when forecasters are very confident of a tornado outbreak - was in 2014.