Tyson Foods Inc. said Sunday that Japan had blocked beef shipments from one of its plants because they did not comply with safety standards designed to guard against mad cow disease.

Tyson said it had inadvertently included in a shipment to Japan a box containing spinal bones, which are prohibited under a trade agreement.

Tyson plans to work with the U.S. Agriculture Department to investigate the cause of the mix-up at its Lexington, Nebraska beef plant, spokesman Gary Mickelson said.

The company said it has seven other plants that continue to be approved to ship to Japan.

Under a trade agreement with Japan, U.S. exporters must remove spinal columns, brains and other cattle parts that are associated with the disease.

The mix-up comes as U.S. officials hope to negotiate an end to certain restrictions put in place for beef exports after the first cases of mad cow disease in the United States.

Japan was the top export market for U.S. beef before the United States found its first case of mad cow disease in December 2003. Tokyo first banned U.S. beef, but since 2006 has allowed imports of been from animals under the age of 20 months. U.S. cattle producers hope to raise that age limit to 30 months to allow for exports of a broader range of products.

Tyson said its shipment, a 35-pound box of short loins, complied with the age requirement and was safe to eat.

(Reporting by Andy Sullivan; Editing by Diane Craft)