At least 11 people were killed as tornadoes ripped through Texas, according to authorities who searched house-to-house for additional victims of the freak storms lashing the southern United States.

The rare December twisters that flattened homes and caused chaos on highways raised the death toll from days of deadly weather across the South to at least 28.

The extreme weather, fueled by unseasonably warm air, is likely to continue for the next few days, the National Weather Service warned, complicating search and rescue efforts and possibly wreaking more havoc in the region.

Texas Governor Greg Abbott on Sunday declared a state of emergency across four counties, including the densely populated Dallas area where several tornadoes touched down late Saturday, the day after Christmas.

In the hardest-hit Dallas suburb of Garland, authorities confirmed eight fatalities after a tornado packing winds of up to 200 miles per hour bore down on the city.

Aerial footage taken as day broke showed some homes completely flattened, while others had roofs blown off and windows shattered, curtains fluttering in the wind.

No official overall toll is yet available for the state of Texas, as emergency crews continue to search through the debris for possible victims.

"We have search teams from the state level, from other cities, from our Texas task force combing through," the mayor of Garland, Douglas Athas, told CNN. "We're going to look at every house and every car to try to make sure we find everybody."

About 600 buildings have been damaged across a two-square-mile (five-square-kilometer) area in Garland.

"I've seen the damage. It's extensive," Athas said. "Cars look like they've been brought back from a war zone and put on display, upside-down."

A statement on the Garland city website said National Weather Service meteorologists found the damage consistent with a category four tornado on the Enhanced Fujita scale.

"EF-4 events have a wind velocity between 166-200 mph, and make up less than one percent of all tornadic activity. This is the second EF-4 tornado in Dallas County since 1950," it added.

Police said the deaths in Garland happened during tornado-related traffic accidents near Interstate 30 and the George Bush Turnpike, the Dallas Morning News reported, saying some of the bodies were found in cars while others were catapulted from the scene.

Three other storm-related fatalities occurred in the towns of Copeville and Blue Ridge in Collin County to the northeast of Dallas, the local sheriff's office confirmed to AFP, without providing more details.

The Dallas Morning News reported that an infant was among the dead.

DANGEROUS WEATHER TO CONTINUE

The late Saturday deaths in Texas came as millions of residents in the southern United States struggle to recover from fierce storms and heavy flooding, with more extreme weather forecast.

Governor Abbott said the northwest of the state was battling harsh wintry conditions, with snow and ice causing power outages.

Central Texas meanwhile was facing flood risks, while the east was bracing for the possibility of more tornadoes.

"We need you to remain vigilant," he told a news conference, urging residents to take cover, and stay off the roads if possible.

The National Weather Service warned of "blizzard conditions" from west Texas into Kansas, and "hazardous ice accumulations" in Oklahoma.

"A variety of dangerous weather conditions will continue across the middle of the country through Sunday," it said. "Dangerous flooding will extend from north Texas to central Illinois."

Flood warnings and advisories also remained in effect in parts of Mississippi, Tennessee, Alabama, Kentucky and other areas in the southeast.

At least 17 people have been killed in storm-related incidents since Wednesday: 10 in Mississippi, six in Tennessee and one in Arkansas, local officials said.

In Alabama, flooding continued Sunday following several days of heavy rain that began on Thursday.

Governor Robert Bentley declared a state of emergency to deal with the flooding just before tornadoes uprooted trees and tore off rooftops on Christmas Day.

Near the state capital Montgomery, more than 300 inmates at the minimum-security Red Eagle Community Work Center were forced to evacuate due to flooding, local media reported.

And residents of the town of Elba were nervously eyeing a levee amid forecasts that the Pea River would possibly overflow the barrier, the AL.com news site reported.

In Mississippi, where Governor Phil Bryant declared a state of emergency to deal with flooding, "severe storms" are forecast for late Sunday through Monday, the state Emergency Management Agency said.