The boxes containing 53 kilograms of ribs were part of a 15.2 ton shipment that arrived in Busan on May 25, the National Veterinary Research and Quarantine Service said.

It said since South Korea has banned imports of U.S. bone-in beef, the ribs will be sent back or destroyed.

"The two boxes were packed full of chuck short ribs so they were easily noticed by our inspectors," said Kang Mun-il, head of the state-run service under the Agriculture Ministry.

He said Seoul immediately suspended imports from the meat processing company that shipped the rib-filled boxes, and that the ban will be maintained until a detailed investigation of the incident is concluded.

The decision to halt imports from the meat processing company has been forwarded to the U.S. government.

The director general, however, said tests on the bones showed they were not tainted with specified risk materials (SRMs). SRMs, which pose the greatest risk of transmitting mad cow disease to humans, include head bones, brains, vertebral columns, spinal cords, dorsal root ganglions and certain internal organs.

South Korea banned imports of beef from the U.S. in December 2003 after a case of mad cow disease was reported in that country. Before the ban, Korea was the third-largest importer of U.S. beef after Japan and Mexico. Of the imports, roughly 60 per cent were bone-in beef like ribs.

Source: Yahoo Finance