ST. LOUIS -- The nation's worst drought in decades is showing no sign of letting up in several key Midwest farming states, worrying farmers harvesting the summer's withered corn crop in record time that their winter crops may also be at risk.

Southeast Missouri, meanwhile, continued to be much wetter than at the height of the summer, when the drought was at its worst locally.

Overall drought conditions in the lower 48 states held steady over the seven-day period ending Tuesday, with about one-fifth of the total land area in extreme or exceptional drought, the two worst classifications, according to the U.S. Drought Monitor's weekly update of its drought map released Thursday.

Southeast Missouri, which spent much of the summer in extreme or exceptional drought, remained mostly unchanged since last week. Most of the area was still in a state of moderate to severe drought, a lower level of drought than extreme or exceptional. Some parts of the extreme Bootheel remained in extreme drought.

Conditions worsened, though, in Kansas and Iowa, the nation's biggest corn producer, and nearly 98 percent of Nebraska was still deemed to be in one of the two worst categories.

The unrelenting dryness won't have much effect on the region's corn and soybean crops, which are already being plucked from the fields. But it could hurt other crops, such as winter wheat.

According to the map, which is put out by the National Drought Mitigation Center at the University of Nebraska in Lincoln, 75 percent of Iowa is enduring extreme or exceptional drought. That's up roughly 10 percentage points from the previous week.

Just over 93 percent of Kansas was in the same predicament, which was an increase of roughly 5 percentage points.

As of Monday, 54 percent of the corn crop had been brought in from the fields -- the fastest pace in at least 17 years due to early planting and nearly three times the previous five-year average of 20 percent by this time, the U.S. Department of Agriculture reported. Some 56 percent of the corn crop in Iowa has been harvested, while Illinois has brought in 71 percent and Missouri 88 percent.

Half of the U.S. corn crop is classified as being in poor or very poor shape, essentially unchanged from a week earlier, the USDA said. A year ago, 20 percent of corn in the fields was listed that way.

Forty-one percent of the U.S. soybean crops have been harvested -- double the pace of the average of the previous half decade -- with one-third considered poor or very poor, the USDA said.

The USDA reported Monday that emergence of winter wheat was lagging, given the extremely dry conditions that could keep that rotational crop from properly germinating. Just five percent of that crop had emerged in South Dakota, down sharply from 32 percent over the previous five years. Similar issues were reported in Nebraska, Colorado, Montana and Oregon.