Chicago - Soldiers and volunteers have packed sand bags in a frantic effort to stave off floodwaters in the US state of Missouri, where 13 people have been killed and several towns have been engulfed.
The Mississippi River is already more than 4.2m above flood stage in some areas and is forecast to rise another couple of metres before cresting on Friday, according to the National Weather Service.
"We've never seen water this high," said Missouri Governor Jay Nixon. "We're in a massive flood fight."
Don Smith, the mayor of Rockaway Beach, a small resort town in the southern part of the state, called the flooding "absolutely devastating" and pleaded for help.
"I don't even know how we're going to deal with the clean-up," Smith said. "One of the business owners has an antique shop: there were minnows inside."
The images shown by local media were dramatic.
Muddy water inundated homes and businesses nearly to the roof. Hundreds of volunteers formed human chains to spread sandbags on levees before the rapidly rising water could overflow them.
A man and his dog were rescued by boat from the roof of a home that was almost completely submerged near the Missouri town of Eureka. Another man was towed out of the floodwaters with his legs dangling out the back window of his pickup.
Nixon urged residents to stay out of the frigid and fast-moving waters. Most of the 13 victims in Missouri died when their cars were swept away.
"It's cold," the governor said. "It's dangerous."
President Barack Obama called Nixon from Hawaii, where he is on vacation, to offer federal assistance if it is needed.
The United States has been hit by a wave of wild weather - tornadoes, floods and rain - that has claimed at least 51 lives in the past week and stranded millions trying to get home after the Christmas holiday.
So far, the flooding in Missouri has only affected small towns and rural areas.
But the Mississippi is forecast to approach or even exceed record highs in the heavily-populated St. Louis area on Thursday and Nixon said he is "very concerned" about the safety of residents there.
"You don't know where that water is going to go," he said.
It could take about a week for the river to drop back down below flood level, the weather service forecast.
The flooding began last week after a massive storm system dumped as much as 25cm of rain in some parts of the state.
Nixon declared a state of emergency on Sunday and called in the National Guard on Tuesday to help local officials deal with the rare winter flooding - the result of a monster storm system that also unleased tornadoes and freezing rain.
Neighbouring Illinois has also been hard hit. The storm claimed the lives of five people swept away while driving on a flooded roadway and a state of emergency was declared in seven counties.
The wild winter weather has killed 11 people in Texas, 11 people in Mississippi and six in Tennessee.
Alabama and Arkansas each reported two storm-related deaths while Georgia blamed one death on the nasty weather.
More misery came on Wednesday as heavy rain led to renewed flash flood warnings in Alabama, Florida, Georgia, North and South Carolina and West Virginia.
"Numerous roads, bridges and underpasses are being threatened by heavy rain and rapidly rising streams and creeks," the weather service warned.
Flooding was also reported in Kansas, Oklahoma, Texas, Louisiana, Mississippi, Arkansas, Kentucky, Tennessee, Indiana and Ohio.