A federal investigation into Syngenta's unapproved corn strain will be completed soon, said Jim Rogers, spokesman for USDA's Animal and Plant Health Inspection Service. Syngenta could be fined up to $500,000 for the incident, USDA said.

The government did not recall the product or make a public announcement about the incident because it was not a food safety risk, according to Rogers.

Syngenta said all of the problematic plantings and seed stock have been "identified and either destroyed or isolated for future destruction."

The company's announcement came as the European Union was pushing ahead with approving more genetically modified crops in the face of some stiff consumer opposition to what has been dubbed there as "Frankenfoods."

Syngenta said seed produced from the contaminated lines over the four-year period represented "one-one hundredth of 1 percent of the US corn acres planted during that time," or 37,000 acres.

While the Bt10 biotech corn strain was mistakenly used, "there is no health or safety issue with this product," the company said. The maize is engineered to resist the corn borer insect.

Syngenta spokeswoman Sarah Hull told reporters the company had discovered the problem in mid-December. At least three US government agencies were investigating the incident.

"Syngenta recently discovered that event Bt10 was present in a very small number of its Bt11 corn breeding lines," according to a company statement.

While Bt10 has not been approved by the government, Bt11 has been approved for distribution for food and feed use and for cultivation in the United States, Japan, Canada and other countries, according to the company. Syngenta says Bt11 and Bt10 have identical characteristics.

While the company said it was "extremely unlikely," it added that some of the contaminated harvested grain "could have entered US export channels as Bt11 through the normal process of grain exports."

Consumer groups expressed concern that more unapproved biotech crops could find their way into the food supply.

"The duration of the violation and potential acreage involved is extremely troublesome," said Greg Jaffe, a biotech expert at the Center for Science in the Public Interest.