Further tests are planned to confirm the diagnosis because the animal had previously tested disease-free, said John Clifford of the US Department of Agriculture (USDA).

"Because of the conflicting results, ... a sample from the animal will be sent to the ... reference laboratory for BSE in Weybridge, England," he said. The USDA will also be conducting further testing, which will take several days to complete, Clifford said in a statement, acknowledging that procedural changes were possible.

"After we receive additional test results on this animal, we will determine what further steps need to be taken, and what changes, if any, are warranted in our surveillance program," he said.

Beef is the number-one protein consumed in the United States, government data show.

A confirmation would bring to two the number of known cases of bovine spongiform encephalopathy (BSE) in the United States, where a diseased animal was discovered in the northwestern state of Washington in 2003.

That discovery prompted Japan, previously the top market for US beef, to halt imports of cattle products. The new case is likely to deepen woes for US cattlemen and fan fears that the deadly human form of the brain-wasting disease could spread in the United States.

The announcement came the same day that Portugal announced its first suspected case of the human form of mad cow disease, while France said it had identified its 13th case of the degenerative brain ailment.

A total of 177 people have died or been diagnosed with variant Creutzfeldt-Jakob disease (vCJD), most of them in Britain, according to a toll compiled by AFP from official figures.

Britain was the epicenter of the BSE outbreak that occurred in the late 1990s. Its suspected source was cattle feed that came from cows with brain disease. Experts believe the pathogen leapt the species barrier to humans through the consumption of contaminated beef.

The animal that tested positive in the United States was unable to walk -- a so-called "downer" animal -- and thus was banned from human consumption, Clifford said. There is no chance its meat entered the human food supply, he stressed.

The bovine died in Texas in November, according to The Washington Post.

Officials did not indicate the age of the animal or whether it was imported, the daily said.

If it was born after 1997, when the United States banned the use of cattle feed containing animal parts, that could call the effectiveness of the ban into question.

The animal that tested positive in 2003 was imported from Canada, and was born prior to the feed ban. Three cases of BSE have been discovered in Canada since 2003.

The United States has applied intense pressure on Japan to resume imports of US beef, but Japan has so far resisted the call amid further safety assessments.

Tokyo said in October it would exempt US cows aged 20 months or younger from screening if high-risk parts such as brains and spinal cords were removed. But the main dispute remains over how to verify the age of the cattle.

Japan is the only Asian country to have confirmed cases of BSE. Its first case was discovered in September 2001, prompting the government to introduce measures to screen every cow slaughtered for consumption.

The USDA, which has resisted international calls to do the same, said Friday it had tested more than 375,000 animals for BSE since June 2004.

Three animals whose tests were inconclusive were subjected to immunohistochemistry (IHC) testing, with negative results, the USDA said.

Earlier this week, however, the deparment's inspector general recommended further testing of the meat samples using a different method, called the SAF immunoblot, or Western blot, test.

"We received final results a short time ago. Of the three samples, two were negative, but a third came back reactive," Clifford said.

So far, 150 people have died of vCJD in Britain, where another six people are living with the disease, according to the official British vCJD website.

There have been two cases in Ireland, with single cases reported in Canada, Italy, Japan, the Netherlands and the United States.