Japan will not impose a blanket ban on U.S. beef imports, despite the discovery of a spinal column in a meat shipment from a U.S. processing plant, a government spokesman said Thursday.

Japan this week temporarily halted beef shipments from a U.S. plant after finding the spinal column, which violated a trade accord prohibiting parts believed to pose a risk of mad cow disease.

But Chief Cabinet Secretary Nobutaka Machimura said the violation was apparently an isolated case, and Tokyo would not retaliate by blocking all beef shipments from the United States.

"We understand this is not a systematic problem concerning U.S. exports to Japan and there is no need to impose an import ban," he told reporters.

"But this is clearly unwelcome, and we asked the U.S. government to fully abide by conditions concerning Japan-bound shipments," Machimura added.

The spinal column was discovered Monday at a Japanese meat-processing factory during an inspection, said a statement from the Ministry of Agriculture, Forestry and Fisheries and the Ministry of Health, Labor and Welfare.

It was found in one of 700 boxes shipped from National Beef California LP and imported by trading house Itochu Corp., the statement said. Shipments from the U.S.-based company have been halted.

Japan imposed a ban on U.S. beef imports in December 2003 after the first case of mad cow disease was found in the United States. The ban was lifted in 2005, but imposed again in January 2006 after an import violation.

U.S. beef imports resumed in July 2006, but sales are a fraction of what they used to be.

Japan's finicky consumers are sensitive to suggestions American meat is unsafe, and some officials urged U.S. producers to be more careful.

"This should not happen again," said Health Minister Yoichi Masuzoe. "We will continue to make maximum measures to ensure food safety."

The U.S. Embassy said Wednesday an investigation would be conducted to find out what happened, but added that the meat shipment itself was completely safe.

Under the agreement, Japan accepts only meat from cattle 20 months of age and younger, which are thought to pose less of a risk of the disease. U.S. exporters must also remove spinal columns, brain tissue and other materials from shipments bound for Japan.

Mad cow disease, formally known as bovine spongiform encephalopathy, is a degenerative nerve disease in cattle. In humans, eating meat products contaminated with the illness is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady