* China buys up to 600,000 tonnes U.S. soy -traders
* Analysts surprised by USDA soy, corn stocks data
* USDA cuts estimate for 2018 soybean crop (New throughout, adds USDA data, Chinese purchases of U.S. soybeans, changes byline/dateline from LONDON)
CHICAGO, Sept 30 (Reuters) - China bought more American soybeans on Monday as the U.S. Department of Agriculture reported stockpiles of the crop were smaller than traders expected, sending futures prices to nine-week highs.
Corn futures approached a seven-week high after the USDA report also showed tighter-than-expected inventories of the yellow grain.
Farmers welcomed the rallies after crop prices plunged last month when the federal agency estimated that this autumn's harvests will be larger than many projected. They have struggled due to low prices, the U.S.-China trade war and historic springtime flooding, which caused unprecedented delays in corn plantings.
China, the world's largest soybean importer, slashed U.S. imports after the start of the trade dispute last year. Chinese buyers have recently made a string of purchases, though, including up to 600,000 tonnes on Monday, according to traders.
"We're seeing China business every day," said Ted Seifried, chief market strategist for Zaner Ag Hedge in Chicago.
Most actively traded soybean futures were up 2.4% at $9.04 a bushel at the Chicago Board of Trade by 12:45 p.m. CDT (1745 GMT). The most-active corn contract was up 3.3% at $3.83-3/4 a bushel. Most-active wheat futures were up 2.3%at $4.98-1/4 a bushel.
The USDA estimated U.S. soybean supplies as of Sept. 1 at 913 million bushels, below the average estimate for 982 million bushels. Soy stocks, however, were up 108% from the same point last year, due largely to the drop in U.S. exports to China.
The agency cut its estimate for the 2018 soybean crop by 2.6% to 4.428 billion bushels, which contributed to stockpiles being smaller than expected. That was the largest-ever revision to the soybean crop in any September USDA quarterly stocks report, said Bill Lapp, president of consultancy Advanced Economic Solutions.
Corn stocks were 2.114 billion bushels, the USDA said, below projections for 2.428 billion and down 1% from a year ago. Wheat stocks were slightly above trade expectations at 2.385 billion bushels.
"We were surprised that USDA didn't make an adjustment in U.S. corn production for 2018 because stocks came in much below expectations," said Terry Reilly, senior analyst for Futures International in Chicago.
The United States still has ample supplies in storage, traders said. The outcome of the U.S.-China trade war and the size of the autumn harvests are also uncertain.
"We're sitting on a lot of soybeans out there, and it's going to take a long time to work through this stockpile," Reilly said.
(Reporting by Tom Polansek in Chicago Additional reporting by Nigel Hunt in London and Colin Packham in Sydney Editing by Kirsten Donovan and Matthew Lewis)