More study on the risk of imported U.S. beef being infected with mad cow disease is needed before reopening the Japanese market, a food safety panel said Wednesday.

A Food Safety Commission special panel on mad cow disease -- bovine spongiform encephalopathy, or BSE -- met and examined so-called worst-case scenarios on the chances of importing unsafe American beef, panel spokesman Eisaku Kikuchi said.

Opinion on the panel varied widely over the wisdom of lifting the import ban on U.S. beef, Kikuchi said. Prior to the Dec. 24, 2003, ban, Japan was the most lucrative overseas market for U.S. beef, estimated at about $1.5 billion a year, and Washington has been pushing hard for imports to resume.

The panelists, however, decided they needed to look further into the risks. "It's anybody's guess when that will be," Kikuchi said when asked when the ban might be lifted.

Concerns about the safety of U.S. beef are high in Japan. At the time of the ban, Japan required all its domestic cattle to be tested for mad cow disease before being slaughtered, and had demanded the U.S. impose the same system.

Months of negotiation led to a compromise under which both countries would only test cattle older than 20 months. Experts say that no cow younger than 21 months has ever tested positive for the disease.

American officials are pushing for a quick resumption of imports, but Japanese officials say they need to go through the proper bureaucratic steps -- including deliberations in the Food Safety Commission -- before going ahead.

In the meantime, however, Japan decided in May to waive mad cow tests for Japanese cattle younger than 21 months.