WASHINGTON (Reuters) - Japanese officials want the U.S. Agriculture Department to increase testing for StarLink biocorn because Tokyo continues to find traces of the unapproved variety in American shipments, a USDA official said on Thursday.

Japan, the single biggest buyer of American corn, said last week it found traces of StarLink in one of five samples sent on Jan. 15 by the United States for food use, marking the third such discovery in about a month.

The same samples had tested negative at a USDA laboratory before leaving the country.

U.S. regulators bar StarLink for human use because of concerns the corn, made by France's Aventis SA, is might cause allergic reactions.

The corn is approved for animal consumption in the United States. But in Japan, StarLink is not approved even for animal feed.

Under a U.S.-Japanese weekly testing protocol agreed upon in November, a USDA laboratory in Kansas tests three 400-kernel samples randomly chosen from the domestic corn harvest. The same samples are then sent to Japan to verify the accuracy of USDA testing for StarLink.

John Pitchford, director of international affairs for USDA's Grain Inspection, Packers and Stockyards Administration (GIPSA), said Japanese officials asked the USDA increase its test size from 400-kernel samples to 800-kernel samples to improve accuracy.

Pitchford said the department was considering Japan's request, but did not know when a decision would be made.

"From our standpoint, we feel the protocol is very effective," Pitchford said. "We are confident we will be able to address their concerns."

Pitchford was among a small team of GIPSA officials who traveled to Tokyo last month to monitor how it tests for StarLink.

Meanwhile, Japanese officials arrived in the United States to monitor StarLink testing under the corn-for-feed protocol agreed upon in December. They were expected to meet with USDA officials in Washington on Monday.

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