South Korea ? South Korea has asked the United States to explain why a shipment of American beef rejected for having banned bone fragments also contained unacceptable levels of the toxic chemical dioxin, an official said Friday.

The discovery was the latest bad news for the U.S. cattle industry in South Korea, already dealing with the rejection of three recent shipments of beef for including banned bone fragments, which South Korea fears could potentially harbor mad cow disease.

Officials said the beef with the dioxin was in the third of the shipments, which was rejected Dec. 6.

Seoul has asked Washington to explain why the beef contained the dioxin, said a South Korean Agriculture and Forestry Ministry official, who refused to give his name.

A U.S. government official, speaking Friday on condition of anonymity due to the sensitivity of the matter, defended the safety of U.S. beef and said the Department of Agriculture was working on a request seeking details on South Korea’s testing methodology. He said that dioxin comes in varying levels of toxicity.