Wild weather that saw tornadoes and flooding kill dozens of people continued to advance across the United States Monday, now bringing deep snow and freezing rain over a huge swath of the country.

At least 44 people died over the weekend from tumultuous weather that included rare but extremely powerful December twisters.

On Monday, more than 20 states were under a weather watch or warning from a massive storm that stretched from Texas to as far north as Maine.

The National Weather Service said the northern tip of the storm could bring up to a foot (30 centimeters) of snow as it moved across the Midwest Monday and into New England on Tuesday.

The southern front was forecast to spread freezing rain, thunderstorms and -- possibly even more tornadoes.

Over 2,100 US flights were canceled and another 3,700 were delayed Monday, according to tracking service FlightAware.

Airports were already overwhelmed by frustrated travelers after some 1,640 flights were scrapped and more than 6,400 delayed Sunday as the storm system essentially shut down airports in Texas.

Hardest hit on Monday was the major hub of Chicago, where freezing rain and gusting winds grounded over 1,200 flights, according to FlightAware.

City crews and Chicago residents struggled to clear streets and sidewalks of growing piles of thick slush as strong winds whipped biting pellets of freezing rain in every direction.

Winds gusting as high as 50 miles (80 kilometers) per hour were expected to knock down trees and power lines while heavy accumulations of sleet and ice were also likely.

The massive storm system -- fueled by unseasonably warm air that began in the deep south Wednesday -- had already dumped as much as 41 inches (104 centimeters) of snow in parts of New Mexico and 10 inches of rain in parts of Arkansas, Missouri, Oklahoma, and Texas the weather service said.

The governors of New Mexico, Texas and Missouri declared states of emergency for all or parts of their states Sunday to better handle storm damage.

Alabama, Mississippi and Georgia also took similar measures.

- Deadly Texas twisters -

Meanwhile, hundreds of people in Texas were struggling to rebuild homes damaged or destroyed in the weekend twisters.

At least 11 people were killed in the state over the weekend as tornadoes struck the heavily populated Dallas area.

The rare December twisters flattened homes, knocked cars off highways and flipped big-rig trucks like toys. The NWS said that at least nine twisters touched down in the region late Saturday.

Hardest-hit was the Dallas suburb of Garland, where authorities confirmed eight fatalities after a tornado packing winds of up to 200 miles (320 kilometers) per hour bore down on the city.

City officials said this was only the second time since 1950 that such a powerful twister struck the area.

Aerial footage showed rows of flattened homes, while others had roofs ripped off and windows shattered. Some 600 buildings were damaged or destroyed, officials said.

"The fact that people survived it, it's amazing," Garland police lieutenant Pedro Barineau told CNN Monday.

"It's a resilient city and we have plans in place to make the community strong once again."

US President Barack Obama called Texas Governor Greg Abbott to relay his "heartfelt condolences" over the lives lost, and to those who lost their homes, a White House official said.

- Snow, ice, flooding -

Missouri Governor Jay Nixon said that at least eight fatalities due to flash flooding were reported in his state.

"I urge Missourians in flood-affected areas to stay alert, avoid travel if possible and never drive into a flooded roadway," he said.

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

The agency also reported 10 storm-related deaths.

Illinois blamed five deaths on the weather, while six deaths were reported in Tennessee, two people were killed in Alabama and one person died in Arkansas, local officials said.

Authorities in Georgia on Monday confirmed one man had died when his car was swept away by floodwaters, several US media outlets reported.