Japan and South Korea suspended some imports of US wheat Friday as American regulators investigated the discovery in an Oregon field of rogue wheat genetically engineered to resist Monsanto herbicide.

US regulators insisted that the wheat carries no risks, but the outcome of the probe -- namely finding out whether such wheat is growing elsewhere -- could have a wider impact on world markets, with the US the biggest global exporter of wheat.

"No one wants genetically engineered wheat," said Bill Freese, science policy analyst at the Center for Food Safety, recalling that massive opposition in 2004 led seed giant Monsanto to pull back from its bid to commercialize it.

GE wheat is not approved for commercial sale anywhere in the world, but some herbicide-resistant plants were found in April on an Oregon farm, triggering a government investigation, officials announced on Wednesday.

Market jitters ensued as Japan canceled a bid for 25,000 tons of US wheat, and the European Union told its member states to test imports from the area, saying any genetically modified wheat would not be sold to consumers.

Taiwan's Flour Millers Association said imports from the United States were under review, according to Dow Jones Newswires.

South Korea's millers association also temporarily suspended purchases of soft white wheat from the United States, according to the US Wheat Associates and the National Association of Wheat Growers. Both groups urged nations not to overreact.

"Because of the isolated nature of this discovery, there appears to be little scientific reason for governments to suspend US soft white wheat purchases," the groups said.

"USDA made it clear that there is no evidence suggesting that this material has entered commercial supplies and that there is no health risk associated with it."

Monsanto's share price was down four percent at $100.64 Friday after climbing for the past 10 days.

The altered wheat is glyphosate resistant, which means it contains a transgene that allows it to survive when a popular weed killer made by Monsanto, called Roundup, is sprayed on fields.

The GE wheat was tested at more than 100 sites in the United States from 1998 to 2005, but the last approved field trials in Oregon were in 2001, according to the USDA's Animal and Plant Health Inspection Service (APHIS).

Monsanto said it was cooperating with the probe and vowed to "conduct a rigorous investigation to validate the scope of and to address any presence of a Monsanto Roundup Ready event in commercial wheat seed."

The same GE technology is already widely used in soybeans and maize in the United States, and allows farmers to spray their fields with weed killers without harming the main crop.

"The first thing to know is it is perfectly safe. It was fully tested by Monsanto prior to 2005," said Ronnie Coffman, professor of plant breeding and director of international programs of the college of agriculture and life sciences at Cornell University.

"Wheat farmers have wanted it all along, but there is the sense that the issue among the big wheat exporting nations is sort of, 'You first.'

"The Americans don't want to do biotech wheat before the Canadians do because they are afraid they might lose market share. The Canadians, the Americans, the Australians are all big wheat exporters and are all very sensitive to the market forces."

Consumer advocates said US regulators have failed to protect the food supply from GE crop contamination, noting that Food and Drug Administration safety reviews of GE crops before market release are voluntary.

Freese said the situation in Oregon "shows the total inadequacy of the regulatory system" for GE crops.

Other concerns include the potential human and animal impacts of the rising use of Roundup, with more chemicals being used in the environment because more crops are now modified to be resistant.

The FDA concluded in its 2004 review of Monsanto's research that the GE wheat variety was as safe for use in animal feed and human food as non-GE wheat on the market.

The wheat market could face more pressure as a result of the Oregon incident, according to analyst Frank Cholly of RJO Futures, who said jitters could spread like "a contagion, it may become more widespread."