TOKYO (CNN) - Japan halted the import of U.S. beef Friday after animal spines were found in three boxes of frozen beef at Tokyo International Airport and asked U.S. officials to explain what happened.

Slightly more than a month ago, Japan lifted a two-year ban on U.S. beef amid concerns about mad cow disease. It is unclear how long the temporary stoppage of U.S. beef imports will last.

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The shipment of beef with spinal cords attached violates the agreement between the two countries, meaning a ban could be reinstated.

A Japanese Agriculture Ministry statement said the importation of U.S. beef had been halted until the U.S. government can report to the ministry "the cause of the discovery."

The statement said when 41 boxes of frozen beef arrived from a firm in New York, inspectors found beef with spinal cords attached in three of the boxes.

U.S. Agriculture Secretary Mike Johanns said in a statement Friday morning that a thorough investigation was underway and that the plant that exported the tainted shipment would no longer be able to export beef to Japan.

"Our agreement with Japan is to export beef with no vertebral column and we have failed to meet the terms of that agreement," Johanns said in the statement.

Johanns also said he was dispatching USDA inspectors to Japan to review shipments and that unannounced inspections would take place at every U.S. plant approved for beef exports.

Shoichi Nakagawa, the agricultural minister of Japan said, "If that's true (that spinal cords were found in a beef shipment), we consider that is a grave violation of import process. We will look into it closely and ask U.S. side to clarify the problem."

A short time later, Japanese Prime Minister Junichiro Koizumi, told reporters: "I just heard the news from the agricultural minister just now over the phone, and he said Japan will stop the import totally.

"He also said that it is extremely important to secure the food safety for the Japanese people and we should ask U.S. to have appropriate measures. I told him that it is a good action. I told the agricultural minister that he should consult well with health and welfare minister to take an appropriate action and request the U.S. side appropriate measures."

In December, Japan lifted the ban on U.S. beef, saying new safety measures would decrease the public's risk of contamination due to BSE, or mad cow disease, from American beef. Under the new regulations announced at the time, only cows younger than 20 months could be exported to Japan, and all brain, spinal cord and other material must be removed.

Japan, which has had its own cases of BSE, tests every cow sold and previously had demanded the United States do the same. U.S. producers, however, said such steps were not necessary or economical.

Before the ban, Japan was the most lucrative market in the world for American beef, importing than $1.7 billion worth in 2003, according to the U.S. government.

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