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 on 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.

According to a statement issued late Thursday, the South Korean Agriculture and Forestry Ministry found 6.26 picograms of the toxic substance in one gram of fat, part of a 10.2-ton shipment of U.S. beef which arrived on Dec. 1.

South Korean standards allow no more than 5 picograms per gram of fat. A picogram is equivalent to a trillionth of a gram.

Seoul barred U.S. beef in December 2003 after the first reported U.S. case of the disease, known as bovine spongiform encephalopathy. Imports recently resumed after a nearly three-year ban, but so far no beef has made it to South Korean food stores or restaurants.

Kim Mee-kyung, a senior researcher at the quarantine service, said dioxin is sometimes found in beef due to environmental pollution in the food chain. She said testing for the toxin is carried out at random on about 100 samples of imported beef a year.

South Korea, formerly the third-largest foreign market for American beef, agreed to resume imports earlier this year of boneless meat from cattle younger than 30 months old, citing worries that some material inside bones could be dangerous to consume, and that younger animals are safe from mad cow disease.

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.

U.S. officials have criticized South Korea over the rejections.