The United States put more pressure on South Korea Friday to further lift a ban on imports of American beef - a ban which was imposed because of fears of mad cow disease.

Officials say they are also pressuring Japan to do the same.

The U.S. reported two cases of mad cow disease, the first in 2003 and the second in 2005. Japan has reported 21, however, South Korea has not had any reported cases of the disease.

South Korea agreed Friday to resume imports of U.S. beef, however, they will not accept ribs or any other bone-in meat. This excludes at least 45 percent of the market.

South Korea had been the third largest market for U.S. beef, behind Japan and Mexico, before fears of mad cow disease ceased imports. U.S. beef producers depend on South Korea's $815 million dollar market.

U.S. Trade Representative Rob Portman told the Associated PressAP