Excessive rain late this week will further slow U.S. corn and soybean plantings and there is the risk of additional flooding that could harm crops in low-lying areas, an agricultural meteorologist said on Wednesday.

"It will be wettest beginning Thursday with the heaviest rains in Missouri, Iowa and Illinois," said Don Keeney, a meteorologist for MDA Weather Services.

Keeney said there would be widespread rains of more than 1.0 inch with many areas receiving 2.0 to 3.0 inches or more. It will also rain in the U.S. Central Plains and in the Northern Plains with heavy downpours of 2.0 to 4.0 inches or more on Thursday and Friday.

"Next week there will be more rain in the northwest Midwest but drier in the southern Midwest," Keeney said.

U.S. farmers slowed the pace of planting during the past week due to rainy conditions that delayed the tail end of corn seeding and pushed soybean planting to its slowest in 17 years, the U.S. Department of Agriculture said in its weekly crop progress report on Tuesday.

The slow seeding of both crops this spring has raised concerns about reduced yields at autumn harvest as key phases of crop development will likely be delayed until the heat of the summer. A late planting also increases the possibility of an early frost inflicting further damage on the crops.

The USDA said that corn planting was 86 percent complete as of May 26, up 15 percentage points from a week earlier.

The corn progress was down from 99 percent a year ago and behind the five-year average of 90 percent. But prospects were much improved from just two weeks ago, when muddy fields led to the slowest start on record for corn planting.

Farmers had finished 44 percent of soybean planting as of May 26, compared with 87 percent a year ago and the five-year average of 61 percent. It was the slowest pace for soybeans since 1996, when farmers had seeded just 35 percent of their crop by the end of May.

"The rains fell again across the state last week bringing planting progress to a halt," the Illinois field office of USDA's National Agricultural Statistics Service said in a report.

Analysts had been expecting corn planting to be 86 percent complete and soybean planting to be 42 percent finished, according to the average of estimates in a Reuters poll.

(Additional reporting by Mark Weinraub in Chicago; Editing by Maureen Bavdek)