The request, which the sources said comes with a hint that Washington could file a complaint with the World Trade Organization over the matter, puts pressure on Tokyo ahead of Prime Minister Shinzo Abe's visit to Washington later this month.

During the upcoming talks, President George W. Bush is expected to prod Abe to settle the bilateral row, which traces its origin to the 2003 discovery of U.S.-grown cattle infected with mad cow disease.

Japan, which banned beef imports from the U.S. following the discovery, resumed the imports in December 2005 on the condition that the beef be from cattle aged up to 20 months and that dangerous parts be removed completely.

Washington has since been urging Tokyo to soften the age restriction and is now threatening to take the matter to the WTO, the sources said. Those who have referred to possible WTO action are senior officials in Department of Agriculture, including Secretary Mike Johanns, they added.

Last month, Johanns said Japan should act swiftly to relax its conditions on American beef once the World Organization for Animal Health grants official beef-safety status to the U.S. at its General Assembly meeting in May, as widely anticipated.

U.S. proponents of going to the WTO believe the body would rule against the Japanese restrictions if the animal disease watchdog, known commonly by the French acronym OIE, formally determines that all U.S. beef is safe for export regardless of cattle age, the sources said.

The latest U.S. move came after South Korea agreed to import U.S.-grown beef with bones when it negotiated a free-trade agreement with the United States, though it had banned imports due to the risks of mad cow disease.

Source: The Japan Times