A storm system forecasters called "particularly dangerous" killed at least six people as it swept across the U.S. on Wednesday.

Tornadoes touched down in Indiana and Mississippi, where three people were killed.

A tree blew over onto a house in Arkansas, killing an 18-year-old woman and trapping a one-year-old child inside, authorities said. Rescuers pulled the toddler from the home to safety.

The Tennessee Department of Health has confirmed that two people were killed in severe storms in that state Wednesday. Officials said the two deaths — one male and one female — occurred in Perry County. 

A tornado damaged or destroyed at least 20 homes in the northwest part of Mississippi. Bill Luckett, the mayor of Clarksdale, Miss., said Wednesday a dog was killed by storm debris. Planes at a small airport overturned and an unknown number of people were injured.

"I'm looking at some horrific damage right now," the mayor said. "Sheet metal is wrapped around trees; there are overturned airplanes; a building is just destroyed."

Television images showed the tornado appeared to be on the ground for more than 10 minutes. Interstate 55 was closed in both directions as the tornado approached, the Mississippi Highway Patrol said.

After an EF-1 tornado struck the south Indianapolis suburb of Greenwood, television stations showed pictures of damage including a portion of a roof blown off a veterinary office.

The biggest threat for tornadoes was in a region of 3.7 million people in Mississippi, Tennessee and Arkansas and parts of Missouri, Illinois and Kentucky, according to the national Storm Prediction Center in Oklahoma. The center issued a "particularly dangerous situation" alert for the first time since June 2014, when two massive EF4 twisters devastated a rural Nebraska town, killing two people.

The greatest risk for a few "intense, long-tracked tornadoes" will be through Wednesday night.

About 200 kilometres east of the tornado, Brandi Holland, a convenience store clerk in Tupelo, Mississippi, said people were reminded of a tornado that damaged or destroyed more than 2,000 homes and businesses in April 2014.

"They're opening all our tornado shelters because they say there's an 80 per cent chance of a tornado today," Holland said.

Holiday decorations could become dangerous

Elsewhere, skiers on the slopes out West got a fresh taste of powder and most people in the Northeast enjoyed spring-like temperatures as they finished up last-minute Christmas shopping.

"It's too warm for me. I don't like it. I prefer the cold in the winter, in December. Gives you more of that Christmas feel," said Daniel Flores, a concierge from the Bronx, his light jacket zipped open as he shopped in Manhattan with his three children.

Only about half of the nation, mostly in the West, should expect the possibility of a white Christmas.

In the small coastal town of Loxley, Alabama, Mandy Wilson watched the angry gray sky and told drivers to be careful as she worked a cash register at Love's Travel Stop.

"It's very ugly; it's very scary," Wilson said. "There's an 18-wheeler turned over on I-10. There's water standing really bad. It's a really interesting way to spend Christmas Eve eve."

In parts of Georgia, including Atlanta, a flood watch was posted through Friday evening as more than 100 millimetres was expected, the National Weather Service said.

The threat of severe weather just before Christmas is unusual, but not unprecedented, said Greg Carbin, a meteorologist at the national Storm Prediction Center.

Twisters hit southeast Mississippi exactly a year ago, killing five people and injuring dozens of others. On Christmas Day in 2012, a storm system spawned several tornadoes, damaging homes from Texas to Alabama.

Emergency officials in Tennessee worried that powerful winds could turn holiday yard decorations into projectiles, the same way gusts can fling patio furniture in springtime storms, said Marty Clements, director of the Madison County Emergency Management Agency in Jackson, the state's largest city between Memphis and Nashville.

"If you go through these neighbourhoods, there are a lot of people very proud of what they've put out and they've got stuff everywhere — all these ornaments and deer and everything else," Clements said. "They're not manufactured to withstand that kind of wind speed, so they become almost like little missiles."

In Arkansas, Pope County Sheriff Shane Jones said the 18-year-old woman was killed when a tree crashed into her bedroom. The woman and her 1 1/2-year-old sister were sleeping in a bedroom of the house near Atkins about 100 kilometres northwest of Little Rock, when winds uprooted the tree that crashed through the roof.

Storms South

High winds and heavy rain uprooted a large tree outside of Atkins, Ark., on Wednesday. It fell on a house, killing a woman, 18, and trapping a toddler inside. (Pope County Sheriff's Department photo/AP)

"It's terrible that this happened, especially at Christmas," Jones said.

Forecasters said by Wednesday night, the severe weather threat could shift east into the southern Appalachian Mountain region.

Once the strong storms clear out, warm temperatures were expected. Highs in Atlanta and Charlotte, North Carolina, on Christmas Eve were forecast to be over 20 C.