Rescue workers and heartbroken residents on Friday sifted through what was left of homes wiped out by several ferocious storms and tornadoes that killed at least 14 people in the US southeast.

The storms, feeding on unseasonably warm air, left a trail of destruction in rural communities from Alabama to Illinois, just as Christmas reached its crescendo.

More than a dozen tornadoes were reported in six states, with the southern state of Mississippi hardest hit. Seven people were confirmed dead there and another 60 injured, with one person missing, said the state's emergency management agency.

"We are experiencing some flash flooding today, with storms coming through right now in five counties. And damage assessments are still ongoing," the emergency agency's Brett Carr told AFP.

Among the Mississippi dead was a seven-year-old boy who was killed when a brutal storm picked up and tossed the car he was travelling in, fire chief Kenny Holbrook told reporters in the town of Holly Springs, where thousands greeted Christmas Day without power.

Mississippi Governor Phil Bryant declared a state of emergency in seven counties after the storms caused widespread damage.

Six fatalities were confirmed in neighboring Tennessee, including three people found dead Thursday in a car submerged in a creek, according to the fire department in Columbia, Tennessee.

One person was reportedly killed in Arkansas.

Georgia's governor declared a state of emergency in counties affected by the severe weather.

Debris from ravaged buildings and other structures littered roads, making them impassable in parts of the southeast.

Officials were inviting volunteers to help clean up or make donations as people who fled returned to their homes to see what, if anything, was still standing.

Television footage and pictures posted on social media showed homes flattened across several states, with possessions and Christmas presents strewn on the ground or left in a messy heap.

Power lines, trees and mobile phone towers were also toppled.

Therese Apel, a reporter at Mississippi's Clarion-Ledger newspaper, spoke with a north Mississippi family hard hit by the violent weather.

"The Wilkins family lost everything, but they told me, 'It's still Christmas. It's about family and being grateful,'" Apel posted on Twitter.

The worst appeared to be over, but forecasters at the National Weather Service warned severe weather was possible in several states including Mississippi, Missouri and Oklahoma. Flash floods in the US southeast were also a possibility.

The East Coast meanwhile was enjoying unseasonably warm weather, with temperatures in New York's iconic Central Park peaking at 72 degrees Fahrenheit (22 Celsius) Thursday, the warmest Christmas Eve since records began in 1871.

The location set a Christmas Day record with temperatures reaching 66 degrees (19 Celsius) on Friday.

Atlanta, Georgia was also expected to set a record with temperatures reaching 75 degrees Fahrenheit (24 Celsius).

The opposite was happening in other areas of the country.

"Ho Ho Ho! Vegas got snow!" reported the National Weather Service before dawn Friday, saying the trace of white tied the record set in 1941 for Christmas Day snow in normally balmier Las Vegas. That record was also tied in 1988 and 2008.