CHICAGO (Reuters) - Japan and South Korea have both taken steps to block certain imports of U.S. wheat after unapproved genetically-modified (GMO) plants from Monsanto Co seeds were found growing in Washington state, a spokeswoman for the U.S. Department of Agriculture (USDA) said on Monday.

A combine drives over stalks of soft red winter wheat during the harvest on a farm in Dixon, Illinois, July 16, 2013. REUTERS/Jim Young/File Photo

Japan’s Ministry of Agriculture, Fisheries and Forestry said it will suspend purchases of all Western White wheat from the United States for food use, and all purchases of Western White wheat from the U.S. West Coast, but not from the Gulf, for feed use until it can start testing incoming shipments, according to the USDA.

Japan has also suspended distribution of all previously purchased U.S. wheat until testing is established. It is the first time Japan has blocked U.S. wheat imports since mid-2013, when a different strain of GMO wheat - also developed by Monsanto, but never commercially released - was found on a farm in Oregon.

South Korea has suspended clearance of U.S. wheat for food use, the USDA said. South Korea, the fifth largest market for U.S. wheat, had already said it would step up quarantine measures for U.S. milling and feed wheat shipments.

In 2013, some Asian countries along with Japan halted U.S. western white wheat imports for months. In 2014, another group of biotech wheat plants - also developed by Monsanto - were discovered near a Montana State University crop research facility.

Since then, Japan and other wheat importers regularly test shipments of U.S. wheat for biotech traits.

Japan expects to lift its new restriction once it has a system in place to test grain for the new GMO trait seen in the Washington state wheat plants, the USDA said.

“We knew they (Japan and South Korea) were going to be cautious about this until they had the test in hand,” said Steve Mercer, spokesman for trade group U.S. Wheat Associates.

The USDA expects the latest suspensions to end quickly because testing supplies, which include wheat samples sent by Monsanto, should arrive in Japan and South Korea in the next few days, the USDA spokeswoman said.

U.S. federal and state agriculture officials have confirmed an inquiry is under way to figure out how the 22 unapproved GMO wheat plants came to be growing in an unplanted Washington state farm field.