CHICAGO (REUTERS) - The US farm belt remained locked in a deep freeze on Tuesday with sub-zero (Fahrenheit) temperatures, likely damaging portions of the region's dormant winter wheat crop, agricultural meteorologists said.

The latest Arctic outbreak, following weekend snowfall, has also slowed the movement of grain and livestock through the region. Barge traffic has slowed on the Mississippi and Illinois rivers, major arteries for supplying grain to exporters at the US Gulf.

Temperatures dropped to about minus 5 degrees Fahrenheit (minus 20.5 deg Cel) overnight in the southern Midwest, areas where crops lack a protective layer of snow cover.

"There is definitely some winterkill in central Illinois, and a little bit in west-central Indiana, northern Missouri and east-central portions of Nebraska," said Mr Don Keeney, a meteorologist with MDA Weather Services.

"It's not huge amounts of damage. I'm saying less than 10 per cent of the belt," he said.

Another private weather service, the Commodity Weather Group, reported "limited winterkill damage" in nearly 5 per cent of the Plains and Midwest wheat belts, with the hardest-hit areas in central South Dakota, northeast Missouri and central Illinois.

Some limited relief is in the offing this week, with temperatures expected to rise in Chicago by Thursday, Mr Keeney said.

A series of storm fronts on Thursday through Saturday should bring snow to northern Illinois, Wisconsin and Michigan, as well as Iowa and northern Missouri, providing insulation to crops in those areas. But the precipitation will likely miss central Illinois and southwest Indiana.

Another push of cold air follows, sending temperatures in Chicago on Sunday to about 2 F (minus 17 deg Cel), Mr Keeney said.

"We are going to continue to see these cold surges out of the northwest over the next 15 days," he said.

Forecasting models showed storms crossing the southern Plains and Midwest toward the middle of next week that should bring snow to central Missouri and southern Illinois, followed by yet another cold spell.

Farther west, next week's storm should bring much-needed moisture to the southern Plains hard red winter wheat belt, including snow in Kansas and a mix of snow, ice and rain in Oklahoma and Texas.

"Across Kansas, we should see a decent improvement in moisture, even down into Oklahoma and Texas," Mr Keeney said.