CHICAGO, April 18 (Reuters) - U.S. soybean and corn futures fell to six-week lows on Thursday, with both markets well-supplied by South America, despite rising concerns about the size of Argentina's corn harvest, traders said.

Wheat futures firmed, supported as drought intensified in portions of the U.S. Plains winter wheat belt.

As of 12:44 p.m. CDT (1744 GMT), Chicago Board of Trade (CBOT) July soybeans were down 12 cents at $11.52-1/4 a bushel after dipping to $11.50-3/4 a bushel, the contract's lowest since March 1. July corn was down 3-1/4 cents at $4.37-3/4 a bushel.

CBOT July wheat was up 2-3/4 cents at $5.55 a bushel while K.C. July hard red winter wheat futures were up 6-1/4 cents at $5.75-1/4 a bushel.

Soybean futures were pressured in part by an influx of Brazilian farmer sales as the Brazilian real fell this week to its lowest since March 2023, making dollar-denominated soybeans worth more.

"We saw a lot more South American selling because of the decline in the Brazilian real," said Nathan Losey, senior analyst with AgResource Company.

Brisk export demand for Brazilian soy has helped to lift cash prices in recent weeks, analysts said, while curtailing demand for U.S. supplies. The U.S. Department of Agriculture on Thursday reported export sales of U.S. soybeans in the week ended April 11 at 485,800 metric tons, in line with trade expectations.

Corn futures drifted lower but traders were monitoring the spread of corn stunt disease in Argentina, the world's No. 3 exporter of the feed grain.

The USDA's attache in Buenos Aires lowered its unofficial estimate of Argentina's 2023/24 corn crop to 51 million metric tons, below the USDA's official estimate of 55 million, citing the disease, and a Rosario grains exchange analyst said estimates of the crop could be reduced further.

Wheat futures firmed, bucking the declines in soy and corn.

"We're seeing a bit of strength in the hard red wheat markets as dry weather takes its toll on the Plains crop," Arlan Suderman, chief commodities economist with StoneX, wrote in a client note.

The USDA reported that 24% of the U.S. winter wheat crop was located in a drought area as of April 16, up from 18% a week earlier. However, said Suderman, "cheap Black Sea exports continue to keep a lid on the market." (Reporting by Renee Hickman; Additional reporting by Peter Hobson in Canberra and Sybille de La Hamaide in Paris; Editing by Sherry Jacob-Phillips, Will Dunham and Subhranshu Sahu)