Swollen rivers in the U.S. Midwest and other regions brought flood warnings for over 12 million Americans on Wednesday as scores of buildings were submerged after days of intense rain in which 24 people have died.

Two rivers west of St. Louis crested at historic levels, flooding local towns, disabling sewer plants and forcing the evacuation of hundreds of residents.

Other major rivers including the Mississippi are expected to reach record highs as flood waters rush toward the Gulf of Mexico, the National Weather Service said.

The flooding has closed many roads and parts of Interstate 44, a major artery running from west Texas to St. Louis. It poses a threat to livestock and crops in farm areas stretching from Illinois to Louisiana.

Water rose to the rooftops of homes and businesses in Missouri, where Governor Jay Nixon called the flooding "historic and dangerous."Nixon spoke with President Barack Obama on Wednesday and received a pledge of federal support.

About 300 people in Valley Park, Missouri, west of St. Louis, were evacuated in case a levee is breached on the Meramec River, said Chief Rick Wilken of the Valley Park Fire District. Residents in West Alton and Arnold, at the confluence of the Mississippi and Missouri rivers, and the town of Pacific have also been evacuated.

Most evacuees stayed with family or friends or went to hotels, but Allen Irwin, 40, a construction worker, went to one of seven American Red Cross shelters set up in the area. The Valley Park resident said police came around evacuating people and he only had time to grab his two cats, which he put in a plastic tub. 

"I got the cats out and everything else is replaceable," said Irwin, who noted that construction work will be readily available after the flood. If his house was flooded, he said he would "pick up the pieces and start again."

The Humane Society had a trailer at the shelter to take care of Irwin's cats and other pets.

In Tiptonville, Tennessee, residents were watching the Mississippi rise, and some lowland cotton fields had already flooded and farms been evacuated.

"We shut all the floodgates last night here. People near the river already are moving furniture and valuables to a higher ground," said Dewayne Haggard, manager of the food Rite grocery and a member of the Tiptonville City Board. "One day alone, we had 10 inches of rain. Pray for us."

Further south, Louisiana Governor Bobby Jindal declared a state of emergency as the waters moved toward his state. Flash flood warnings were issued for parts of the Carolinas and Georgia.

Some 12.1 million people nationwide are living in areas where there are flood warnings, the National Weather Service said in a statement.

At least 24 people have died, mostly from driving into flooded areas, in Missouri, Illinois, Arkansas and Oklahoma after days of downpours with as much as 12 inches (30 cm) of rain. 

In Eureka, Missouri, along the Meramec River, Mayor Kevin Coffey said a man was rescued from atop the cab of his pick-up truck after spending the night in a parking lot to watch over his gun shop business.

"This is 4 feet (1.2 meters) above the worst flood we ever had," Coffey said after helping to put sandbags around a school. "The town looks like one huge lake."

Historic floods on the Mississippi in 1993, 1995 and 2011 occurred during warm weather, after snow melts in the north. AccuWeather senior meteorologist Alex Sosnowski called it highly unusual to have heavy flooding in winter and said it could presage trouble for the spring.

While the rains have stopped for now, freezing weather is setting in, which will make the cleanup a miserable undertaking, he said.

Agriculture experts said water standing more than a week could kill the soft red winter wheat crop. Export premiums for corn and soybeans were at their highest levels in weeks because of stalled barge traffic on swollen rivers.

Livestock also has been hard hit. About 2,500 hogs drowned in an Illinois barn after a creek overflowed its banks, said Jennifer Tirey, a spokeswoman for the state's Pork Producers Association. 

"There was no electricity and roads were impassable. It was just impossible to get to those pigs," she said.

The U.S. flooding is occurring at the same time as historic El Nino-related flooding across northern England. The El Nino weather phenomenon tends to disturb global weather patterns as ocean water temperatures rise above normal across the central and eastern Pacific, near the equator.

The Mississippi River, the third longest river in North America, is expected to crest over the weekend at Thebes, Illinois, at 47.5 feet, more than a foot and a half (46 cm) above the 1995 record, according to the National Weather Service.

Flood warnings were issued from eastern Oklahoma into southeastern Kansas, southern Missouri, central Illinois and parts of Arkansas, Kentucky, Tennessee, and the Florida panhandle.

Illinois Governor Bruce Rauner, who has been at an undisclosed foreign vacation destination with his family, intends to return to the state Thursday and spend the following days touring the flood-stricken areas, his spokeswoman told Reuters.

By press release, Rauner declared 12 counties disaster areas.