Snow, sleet and hail snarled transportation in large parts of the United States on Monday during one of the busiest travel times of the year, after dozens died in US storms that were just some of the wild weather seen worldwide over the Christmas holiday period.

More than 40 people were killed by tornadoes and floods during the holiday season in the United States, where rare winter tornado warnings were issued in Alabama on Monday.

Alabama, Mississippi and the Florida panhandle were expected to bear the brunt of the of the day's strongest storms, said AccuWeather senior meteorologist Michael Leseney.

As of about 3 p.m. EST (2000 GMT), more than 1,940 US flights had been canceled on Monday, according to FlightAware.com, while another 2,790 delays were reported. Chicago-area airports were worst hit with hundreds of flights canceled as the city was swept by sleet and hail.

More than a foot (30 cm) of snow was forecast for southwestern Wisconsin and southeastern Minnesota, and snow was also falling in Iowa, Nebraska and Missouri.

A flash flood warning was in effect in eastern Missouri and Southern Illinois, the National Weather Service said. Thirteen people died in flash floods in those two states during the weekend.

The US storms came as other countries struggled with extreme weather and stressed holiday infrastructure.

In Britain, hundreds of troops were deployed and a government agency said a "complete rethink" of flood defenses was needed after swathes of northern England were inundated by rivers that burst their banks.

Severe weather also hit parts of Australia, where more than 100 homes were lost in Christmas Day brushfires.

Then on Sunday a freight train carrying sulphuric acid derailed in the Outback, and a Queensland Rail spokeswoman told local media that floods had stopped crews reaching the scene.

'RIPPED OUR WORLD APART'

The bad US weather caused two candidates for the Republican presidential nomination, New Jersey Governor Chris Christie and US Senator Marco Rubio, to cancel campaign events in Iowa.

Winter storms that brought ice and high winds to Oklahoma downed power lines and 54,000 customers were without power on Monday in Oklahoma City and surrounding areas, Oklahoma Gas & Electric said. Local news reports said there were 100,000 without power across the state.

Operators of the Kerr and Pensacola dams, about 160 miles northeast of Oklahoma City, warned they would have to release large amounts of water due to the storm and area residents might be forced to evacuate their homes.

Six tornadoes were reported on Sunday - three in Arkansas, one in Texas, and two in Mississippi.

Texas was cleaning up from weekend tornadoes that killed at least 11 people in the Dallas area and damaged about 1,600 structures and homes. One twister in the city of Garland had winds of up to 200 miles per hour (322 km per hour) and killed eight people, including a 30-year-old woman and her year-old son.

"We are very blessed that we didn't have more injuries and more fatalities," Garland's Mayor Douglas Athas told CNN.

In the Dallas suburbs of Garland and Rowlett, which were devastated by tornadoes on Saturday, many residents turned to social media to tell stories of survival and to ask for help finding lost pets.

"RIPPED OUR WORLD APART"

Briana Landrum posted a photo of her living room couch surrounded by wreckage where her house once stood in Rowlett. Her two cats are missing, she wrote, and the freezing rain has made searching for her "sweet babies" difficult.

"All I remember is the windows all shattering and insulation went everywhere," she wrote. "The roof fell on us one second and the next, it was gone ... The tornado ripped our world apart."

Ten deaths and 58 injuries were reported in Mississippi from the Christmas holiday storms, Governor Phil Bryant said at a news conference. Hundreds of homes were damaged.

In flooded southern Missouri, dozens of adults and children forced from their homes took refuge at Red Cross shelters.

Red Cross spokeswoman Julie Stolting said there was no telling when they might be able to return home. "But we're feeding them, we're sheltering them, we're providing health services," she said.

Some roads still were closed in New Mexico, where storms on Sunday dumped as much as 18 inches of snow on eastern parts of the state. Highways with difficult driving conditions included interstate highways 25 and 10. – Reuters