WASHINGTON — Japan halted all imports of U.S. beef because of mad cow fears yesterday, threatening millions of dollars in American trade and sending officials scrambling to repair delicate business relations.

Japan's sudden order came just six weeks after the country had lifted a two-year ban on American beef. The problem this time: discovery of bone — a mad cow disease risk, Asian countries say — in a shipment of veal from a plant in New York.

Agriculture Secretary Mike Johanns called the problem "an unacceptable failure" to meet Japan's requirements. He dispatched inspectors to Japan and ordered unannounced inspections at U.S. plants.

"We are taking this matter very seriously, recognizing the importance of our beef export markets," Johanns said.

At the White House, presidential spokesman Scott McClellan said, "USDA is taking steps to address this matter."

Japan's discovery was a jarring setback for the U.S. meat industry and the Bush administration, both of which had been optimistic about the prospects of selling more beef in Asia, despite lingering restrictions on U.S. products.

Once the world's biggest customer for U.S. beef, Japan last month ended an earlier ban. It agreed to allow shipments of boneless beef from animals younger than 21 months, a stricter requirement than international guidelines call for.

There was much celebrating at the time. One U.S. group flew in a beef shipment for a banquet in Tokyo with the Japanese food service industry. Hong Kong, South Korea and Singapore quickly followed Japan's lead.

Of the $3.9 billion in global sales of American beef in 2003, Japan accounted for $1.4 billion. The other three countries made up about $911 million; they did not weigh in on Japan's latest action.

For now, American beef is being held at Japanese ports until the United States completes a report, which Johanns said would be delivered "immediately." Japan will decide later whether to impose a ban on further imports, department officials said.

An industry group pointed out that the product Japan found, bone-in veal, is eaten in the United States and considered safe under international guidelines. The veal was from calves less than 6 months old, and mad cow disease hasn't been found in animals that young.

"Despite this shipment, sent in error, the facts are indisputable: U.S. beef and veal remain among the safest in the world," said J. Patrick Boyle, president and CEO of the American Meat Institute.

Restrictions against bone-in beef have remained in Asia because officials fear that marrow and other bone tissue might be dangerous.

The government yesterday barred the plant that sent the shipment, Brooklyn-based Atlantic Veal & Lamb, from selling meat to Japan. Johanns said he also would take action against the department inspector who cleared the shipment. The inspector should have noticed on plant documents that the vertebral column, or the backbone, needed to be removed, he said.

Japanese inspectors found material from cattle backbone in three of 41 boxes in a 858-pound shipment of beef from Atlantic Veal & Lamb. All the beef in the shipment was destroyed. Company officials called it an "honest mistake" and said they misinterpreted the export rules.

Japan's prime minister, Junichiro Koizumi, said the situation was "a pity given that imports had just resumed." He told reporters, "I received the agriculture minister's report with his recommendation that the imports be halted, and I think it is a good idea."

He was referring to Agriculture Minister Shoichi Nakagawa, who had visited Johanns in Washington on Dec. 13.

U.S. beef had begun a limited return to Japanese markets and restaurants. However, according to a Kyodo News survey last month, 75 percent of Japanese were unwilling to eat U.S. beef because of mad cow fears.

Mad cow disease is the common term for bovine spongiform encephalopathy, or BSE. It is a degenerative nerve disease in cattle linked to a rare but fatal human disease, variant Creutzfeldt-Jakob disease. People have gotten it by eating meat or cattle products contaminated with mad cow disease. There have been two cases of BSE in the United States and 21 cases in Japan.