GRAINS-U.S. corn, wheat prices fall ahead of key USDA data

* USDA monthly supply/demand report due at 1330 GMT

* South Korean feedmaker avoids buying U.S. corn

* Slower-than-expected U.S. exports weigh on wheat

(Adds quotes, updates prices, previous New Delhi)

LONDON, Dec 11 (Reuters) - Chicago corn futures fell to a three-week low on Tuesday on analysts' expectations that poor export demand will prompt the United States Department of Agriculture (USDA) to raise its forecast for U.S. stocks.

U.S. wheat prices fell to the lowest level in almost five months, also weighed by weaker-than-expected exports, while soybeans edged higher.

Analysts on average expect USDA to raise its forecast of U.S. corn and wheat ending stocks for 2012/13 but cut soybeans.

The USDA's monthly supply/demand report is due at 1330 GMT.

"It will be interesting to see whether the USDA downwardly corrects its estimate for U.S. corn demand after the most recent export figures proved disappointing," Commerzbank said in a market note on Tuesday.

"By contrast, U.S. ending stocks of soybeans could be reduced as a result of the stable demand," the report added.

Underscoring the concerns, USDA reported export inspections of U.S. corn in the latest week at 7.861 million bushels, below a range of trade estimates for 9 million to 15 million.

"Corn has fallen ahead of the USDA data and fundamentals suggest that the trade is expected to remain in the lower range in the near future," said Jonathan Barratt, chief executive of BarrattBulletin, Sydney-based commodity research firm.

Also, reports that a South Korean feedmaker issued an international tender for corn and feed wheat but excluded corn from the United States, the world's biggest supplier, put some downward pressure on the grain.

Chicago Board Of Trade March corn fell 0.6 percent to $7.25-1/2 a bushel by 1026 GMT after dipping to $7.25, the lowest level for the second month since Nov. 16.

LOST U.S. EXPORTS

Wheat fell ahead of the USDA data, which could project lower U.S. exports as well as higher stocks.

The news that the U.S. won only a small share of purchases by Saudi Arabia and Iraq also added to bearish sentiment.

CBOT March wheat fell 0.7 percent to $8.43 a bushel after touching $8.42, the lowest level for the second month since July 13.

"The USDA are likely to lower U.S. wheat export expectations as current sales pace remains substantially below the USDA's target," Macquarie said in a market note.

"Lost exports from the U.S. will be made up by the likely increases in both Australia and Canada, as both of their crops look to have outperformed the USDA's expectations."

March milling wheat in Paris fell a modest 0.4 percent to 265.25 euros a tonne with the market underpinned by export demand, dealers said.

"European prices are supported by a strong export performance as there has been a decrease in Black Sea region exports in the last couple of weeks," said Commerzbank analyst Michaela Kuhl.

Soybeans were marginally higher with January up 0.02 percent at $14.75-3/4 a bushel with USDA data expected to be supportive.

"We expect the USDA to increase U.S. soybean crush, as both meal and oil exports have been running substantially above the USDA's current target levels," Macquarie said.

(Additional reporting by Mayank Bhardwaj in New Delhi and Sarah McFarlane in London; Editing by Alison Birrane)