SEOUL, May 11 (UPI) -- South Korean officials said they delayed sending inspectors to U.S. slaughterhouses, despite mad cow concerns as U.S. beef hit domestic markets Thursday.

South Korea agreed in April to lift its embargo on U.S. beef imports. Seoul banned U.S. beef imports in 2003 amid concerns shipments contained materials linked to the transmission of bovine spongiform encephalopathy, or mad cow disease.

Officials planned to send inspectors to U.S. meat processing plants Monday to alleviate concerns about beef slated to hit the shelves in South Korea Thursday, the official South Korean Yonhap news agency said.

"The dispatch of inspectors was put off due to differences on the issue of scheduling their visits," said an unnamed South Korean official.

South Korean opposition leaders with the United Democratic Party Sunday said they would not back a pending free-trade agreement unless Seoul re-examines beef import concerns.

"It is totally worthless to debate the ratification of the free trade deal without the renegotiation of U.S. beef imports," said party leader Kim Hyo-seuk.

Seoul said in response it would provide assurances U.S. beef entering South Korea is not contaminated with at-risk materials.