GRAINS-Wheat falls 2 pct on U.S. weather; corn down for 3rd day

* Wheat near 1-week low on improved U.S. weather

* Corn pressured by U.S. Midwest planting prospects

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SINGAPORE, April 21 (Reuters) - Chicago wheat fell more than 2 percent on Monday to its lowest in almost a week, while corn slid for a third consecutive session as improved U.S. weather pressured grain prices.

Soybeans gained around half a percent, rising for four out of five sessions on support from tight U.S. old-crop supplies and strong domestic demand.

Chicago Board of Trade wheat for May delivery slid as much as 2.2 percent to $6.76-1/4 a bushel, its lowest since April 15. The market was trading at $6.80 a bushel by 0239 GMT.

May corn gave up 0.5 percent to $4.92-1/4 a bushel, its lowest since April 2 and May soybeans added 0.4 percent to $15.20-1/4 a bushel.

"The weather is improving for wheat in the U.S. Plains," said Kaname Gokon, general manager of research at broker Okato Shoji in Tokyo. "Corn plantings should get a boost this week because of warmer temperatures in the Midwest."

U.S. wheat jumped almost 5 percent last week on concerns that a recent cold snap in key U.S. growing areas damaged some of the crop and worries that shipments from the Black Sea could be disrupted by political unrest in Ukraine.

At least three people were killed in a gunfight in the early hours of Sunday near a Ukrainian city controlled by pro-Russian separatists, shaking an already fragile international accord that was designed to avert a wider conflict.

The U.S. Agriculture Department said on Thursday that weekly export sales of wheat for the 2013/14 crop year were 438,000 tonnes, topping forecasts ranging from 50,000 to 250,000 tonnes.

Corn prices fell on expectations that warming weather would let farmers pick up their pace of planting this week.

The U.S. soybean market has been underpinned by domestic processors running their plants at high levels despite dwindling supplies.

The USDA said that old-crop soybean export sales totalled 19,200 tonnes, within market expectations.

Still, plans by some Chinese importers to default on U.S. and South American soy cargoes could cap the rally in prices.

For weeks, traders have been bracing for a report that showed large cancellations from China, the world's top buyer of the oilseed.

Chinese buyers may default on a further 1.2 million tonnes of soybeans worth about $900 million being shipped from the United States and South America, to avoid incurring huge losses in a depressed local market, the country's top soy buyer said.

Large speculators cut their net long position in CBOT corn futures in the week to April 15, regulatory data released on Friday showed.

The Commodity Futures Trading Commission's weekly commitments of traders report also showed that noncommercial traders, a category that includes hedge funds, switched to net short position in CBOT wheat and raised their net long position in soybeans.

(Reporting by Naveen Thukral; Editing by Joseph Radford)