Widespread floods hit Missouri, Illinois

At least 13 people have died in Missouri since the weekend, with days of downpours triggering the worst flooding in two decades.

At least 13 people have died in Missouri as heavy rain triggered the worst flooding in two decades. (AAP)

Rain-swollen rivers across Missouri are still rising with widespread flooding forcing major evacuations and closing part of a major highway.

At least 13 people have died in Missouri since the weekend, when days of downpours from a massive winter storm system triggered the worst flooding in two decades, Governor Jay Nixon said on Tuesday.

"It's very clear Missouri is in the midst of a very historic and dangerous flooding event," Nixon said.

Several major rivers and tributaries in Missouri and Illinois were poised to crest at record levels, the National Weather Service said. Some of the rivers would continue to rise until Saturday.

At the confluence of the Mississippi and Missouri rivers, about 32km north of St Louis, residents of the towns West Alton and Arnold were told to evacuate on Tuesday.

Flooding in the middle portion of the Mississippi River and its tributaries may reach the highest levels in recorded history, the forecasting site AccuWeather said.

The US Coast Guard closed an 8km stretch of the Mississippi River near St Louis to all vessel traffic due to hazardous conditions.

The National Weather Service forecast the Mississippi River at the Chester, Illinois, river gauge about 100km south of St Louis would crest at 15.1m on Friday - matching the 1993 record.

The floodwaters have forced the closure of roadways and highways, including a portion of Interstate 44, a major highway that runs from west Texas to St Louis, the Missouri Department of Transportation said on Wednesday.

Sewage has been flowing into the fast-rising Meramec River near St Louis since Monday, when floodwater disabled a sewer treatment plant, the St Louis Post-Dispatch reported.

In Illinois, some inmates were moved out of the Menard Correctional Center, a maximum security prison on the banks of the Mississippi River, and sandbags and drinking water were prepared in anticipation of flooding in lower level cell blocks, Illinois officials said in a statement.

Governor Bruce Rauner on Tuesday issued a state disaster proclamation for seven counties to help with response and recovery.

Nixon called out the National Guard to direct traffic away from closed roads in his state and urged people not to drive in flooded areas. Most deaths in flooding occur when cars are swept away.