Washington ? Senior U.S. officials pressed South Korea on Friday to go further in lifting a ban on importing American beef imposed because of mad cow disease.

The officials are putting similar pressure on Japan, which lifted its own ban last month and helped pave the way for such actions by Hong Kong and South Korea.

South Korea agreed early Friday to resume shipments of U.S. beef, which had been prohibited since the December 2003 discovery of mad cow disease in the United States.

But a prohibition will remain on ribs and other bone-in beef, which keeps closed about 45 percent of the potential market. South Korea was worth a total of $815 million to U.S. producers in the year before the ban. The country once was the third-biggest customer of American beef behind Japan and Mexico.

The agreement is a positive step, but the Bush administration is “extremely disappointed” in South Korea’s restrictions, said U.S. Trade Representative Rob Portman.