A massive winter storm system that whipped up tornadoes, ice and snow from the Gulf of Mexico to the Great Lakes on Christmas Day, killing at least seven people, headed for the northeastern US Wednesday.

The storm snarled holiday travel as 1,500 flights were cancelled and people were warned to stay home rather than brave the strong winds, freezing temperatures and treacherous roads.

The National Weather Service warned of "dangerous travel conditions due to snow and ice covered roads" and said the weight of ice and snow could knock down power lines and trees. Already, more than 200,000 people were in the dark.

The weather service forecast up to 18 inches (46 centimeters) of snow from New York state up to Maine and warned of freezing rain, tornadoes and severe thunderstorms all the way down to the Carolinas.

Areas in the Rocky Mountains were also set to get about a foot of snow from a second storm system on Wednesday.

More than 1,000 US flights had been cancelled by Wednesday afternoon, after 533 were grounded on Tuesday, according to flight tracker FlightAware.

"The biggest factor on both coasts is high winds and winds not aligned with runways," FlightAware chief Daniel Baker said.

"This causes significant capacity constraints that leads to long delays and cancellations."

Scores of homes and businesses were damaged Tuesday after 34 tornadoes were reported in the southern US states of Texas, Louisiana, Alabama and Mississippi.

James Bowman said he was sitting in his living room when a sudden wind rattled his rural Texas home apart Tuesday afternoon.

"The inside of the living room started falling down, so I just sat there in the recliner and then it didn't last but just a few seconds -- then it stopped," Bowman, who was alone at the time, told KTRE news.

"I just thank God that I wasn't hurt and the walls and stuff didn't fall in on me."

The governors of both Mississippi and Alabama have declared a state of emergency.

More than 200,000 people remained in the cold and dark Wednesday after the storm knocked down power lines in Arkansas, Mississippi and Texas.

The regional utility company said it was bringing in outside crews to help but it could be a week before service was fully restored.

"Road conditions are making travel very dangerous and slow," Entergy said.

"In addition, the threat of falling trees and tree limbs poses a substantial risk to our crews who are working to restore power as quickly as safely possible."

Two children were killed after their mother lost control of her vehicle on an icy road in Arkansas on Christmas Day, the state police said.

Another person was killed in the state early Wednesday as a result of the storm, the Arkansas emergency management service said.

A dozen people were hurt in a 21-vehicle pileup caused by icy roads in Oklahoma City that started when a truck jack-knifed on a major interstate and oncoming cars were not able to stop on the icy roadway, the Oklahoman reported.

Two women were killed in separate crashes in the state on Christmas Day, the state highway patrol said.

A man was killed in rural Louisiana when a tree hit his home on Christmas Day, and a Texas man died in similar circumstances in a Houston suburb, local media reported.