* Japan cancels tender to purchase some U.S. wheat
* EU preparing to test incoming shipments
* Importers to seek details from U.S. government (Adds NGO, international trade quotes)
By Risa Maeda and Charlie Dunmore
TOKYO/BRUSSELS, May 30 (Reuters) - Unapproved genetically modified wheat found growing in the United States is threatening U.S. exports of the world's biggest traded food commodity, with Japan stopping a purchase against a backdrop of high consumer sensitivity to gene-altered food.
Japan cancelled a tender offer to buy U.S. western white wheat, while other top Asian wheat importers South Korea, China and the Philippines said they were closely monitoring the situation.
"We will refrain from buying western white and feed wheat effective today," Toru Hisadome, a Japanese farm ministry official in charge of wheat trading, told Reuters.
The world's biggest wheat importer, Egypt, said it had no fears yet over supplies.
The European Union is preparing to test incoming shipments, and will block any containing GM wheat. Chicago Board of Trade wheat futures were down around 1 percent.
GM wheat was discovered this spring on a farm in the west coast state of Oregon, in a field that grew winter wheat in 2012. USDA officials said that when a farmer sprayed the so-called "volunteer" plants with a glyphosate herbicide, some of them unexpectedly survived.
Scientists found the wheat was a strain field-tested from 1998 to 2005 and deemed safe before St. Louis-based biotech giant Monsanto withdrew it from the regulatory approval process on worldwide opposition to genetically engineered wheat.
GMO crops tolerate certain pesticides, allowing farmers to improve weed control and increase yields.
No GM wheat varieties are approved for general planting in the U.S. or elsewhere, the USDA said. The EU has asked Monsanto for a detection method to allow its controls to be carried out.
With high consumer wariness towards genetically-modified food, few countries allow imports of such cereals for direct human consumption.
"The developers of GE wheat have repeatedly said that GE wheat will not contaminate conventional or organic wheat because it is predominantly self-pollinating. Despite these empty promises, GE contamination has happened," Greenpeace International scientist Janet Cotter said.
"The only way to protect our food and environment is to stop the releases of GE crops to the environment - including a ban on field trials."
However, the bulk of U.S. corn and soybean crops are genetically modified.
Wayne Bacon, president of French-based grain trader Hammersmith Marketing said there would be a natural knee-jerk consumer response.
"We all buy things with GM products in it every day, we just don't know about it, but if suddenly we know that the loaf of bread we are buying is made from GMO wheat then it becomes a very negative thing with the consumer."
The latest finding revives memories of farmers unwittingly planting genetically modified rapeseed in Europe in 2000, while in 2006 a large part of the U.S. long-grain rice crop was contaminated by an experimental strain from Bayer CropScience , prompting import bans in Europe and Japan.
The company agreed in court in 2011 to pay $750 million to growers as compensation.
Asia imports more than 40 million tonnes of wheat annually, almost a third of the global trade of 140-150 million tonnes. The bulk of the region's supplies come from the United States, the world's biggest exporter, and Australia, the No. 2 supplier.
The USDA said there was no sign that genetically engineered wheat had entered the commercial market, but grain traders warned the discovery could hurt export prospects for U.S. wheat.
"Asian consumers are jittery about genetically modified food," said Abah Ofon, an analyst at Standard Chartered Bank in Singapore. "This is adding to concerns that already exist on quality and availability of food wheat globally."
European traders said Black Sea and EU wheat was well positioned to benefit in any displaced demand for U.S. grain. But some were more pragmatic on the overall impact.
"Japan is in a position to be selective and to react sharply. It has other suppliers and the financial means to be choosy and pay more if needed. This is not necessarily the case for Egypt which is in a difficult financial situation," a European trader said.
China has emerged as a key buyer of U.S. wheat this year, taking around 1.5 million tonnes in the past two months. Chinese purchases in the year to June 2014 are estimated to rise 21 percent to 3.5 million tonnes, according to the USDA, with most shipments coming from the United States, Australia and Canada.
The Philippines, which buys about 4 million tonnes of wheat a year and relies mainly on U.S. supplies, is waiting for more details before acting, an industry official in Manila said.
"I won't be surprised if other countries start cancelling or reducing their purchases of U.S. wheat, particularly Asian countries, putting pressure on wheat demand," said Joyce Liu, an investment analyst at Phillip Futures in Singapore.
Genetically modified crops cannot be grown legally in the United States unless the government approves them after a review to ensure they pose no threat to the environment or to people.
Monsanto in a statement posted on its website said: "While USDA's results are unexpected, there is considerable reason to believe that the presence of the Roundup Ready trait in wheat, if determined to be valid, is very limited." (Additional reporting by Naveen Thukral in Singapore, Niu Shuping in Beijing, Erik dela Cruz in MANILA, Jane Chung in SEOUL, Yayat Supriatna in JAKARTA, Valerie Parent, Michael Hogan and Sarah Mcfarlane; writing by Veronica Brown; editing by Richard Pullin and Keiron Henderson)