WASHINGTON (Reuters) - The U.S. Agriculture Department, reeling from the discovery of unapproved genetically modified (GM) wheat growing in Oregon, said on Thursday it is working to make "appropriate and validated" tests available to detect the wheat.

Domestic and overseas buyers of wheat are keen to have rapid tests to ensure biotech grain is not in their shipments.

Japan has excluded white wheat grown in the U.S. Pacific Northwest from its regular tenders for the past three weeks, although it has continued to buy other U.S. wheat varieties. South Korea and the E.U. plan to test incoming shipments.

Export demand for PNW-sourced wheat has dried up. In the week ended June 6, the first full reporting period since the Oregon discovery was announced, sales were just 1,280 tonnes (1 tonne=1.102 tons), USDA said on Thursday. That was far below sales averaging 69,000 for the corresponding week over the previous five marketing years, USDA's sales database shows.

The USDA's Animal and Plant Health Inspection Service said that currently, there is no commercially available, validated rapid test for GM wheat.

The Grain Inspection, Packers and Stockyards Administration (GIPSA), another branch of the department, is working on tests to "address market needs," USDA said.

In a release, the service said its investigation into the Oregon wheat find continues. It said no other examples of the unapproved strain have been found, and it has no information that any GM wheat is in commercial supply chains.

Separately, USDA said it continues to issue, upon request by exporters, letters saying simply, "There are no transgenic wheat varieties for sale or in commercial production in the United States at this time."

Monsanto, developer of the wheat, has provided to foreign regulators information on how to test for its genetic modification. But the tests are complex and time consuming.

The unapproved wheat found in Oregon was developed years ago by Monsanto, which abandoned field tests in 2005 because there was no market for the wheat. The unwanted "volunteer" wheat sprouted in a field being held fallow this year.

(Reporting by Ros Krasny and Charles Abbott; Editing by Bob Burgdorfer)