The consecutive rejections are likely to escalate tensions at free trade talks between South Korea and the United States currently underway in Montana. Agriculture is a focus of the talks which cover subjects from drugs to financial services.

South Korea, once the third-largest market for U.S. beef, has decided to return all 22 tonnes of U.S. beef recently received since the country lifted a three-year ban on the meat to the United States due to bone fragments.

The country said in September it would resume imports but place tough checks on products to make sure that parts it deemed as risky, such bones, were not included in the shipments.

"All of the imported beef from that (Iowa) shipment will be disposed (of) or returned to the United States and beef exports from that U.S. plant will be suspended temporarily," the ministry said in a statement.

Quarantine officials found seven bone chips in the cargo of chilled beef that arrived on Dec. 1, the ministry said.

South Korea accepts only boneless beef from cattle up to 30 months of age to guard against mad cow disease.

Source: Reuters