CHICAGO - Floods and tornadoes killed at least 43 people in the southern and midwestern United States during the Christmas holiday, with forecasters warning that blizzards and freezing rain would add to the misery on Monday.

Hundreds of people in Texas were struggling to rebuild homes damaged or destroyed by rare but extremely powerful December twisters as residents of Alabama and Florida were warned they could be next.

Millions of people trying to get home after Christmas were stranded by the bad weather, which grounded flights and buried major highways and roadways in snow, ice and floodwaters.

Officials pleaded with people to be wary of flash flooding after several cars were swept off roadways and to be wary of slick conditions following freezing rain.

Over 1,100 US flights were canceled and another 1,650 were delayed Monday morning, 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 450 flights, according to FlightAware.

The National Weather Service said more than 20 states - from Texas to as far north and east as Maine - were under a weather watch or warning. Over a foot (30 centimeters) of snow was expected in some areas and flash flooding and freezing rain in others.

The powerful 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.

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

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

Three other storm-related fatalities occurred northeast of Dallas, the Collin County sheriff's department told AFP.

Governor Greg Abbott told reporters that snow and ice caused power outages in northwest Texas, while central parts of his state were facing flood risks and the east was bracing for the possibility of more tornadoes.

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" are 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.