A single Holstein on a Washington state farm has tested positive for mad cow disease, marking the disease's first appearance in the United States and setting off a chain reaction of countries banning US imports of beef.

The Bush administration assured Americans their food is safe.

But the impact overseas was immediate with several nations, including Japan, South Korea, Australia and Taiwan, halting imports.

They made their announcements just hours after the US Agriculture Department confirmed that a cow at a farm near Yakima, Washington, had tested positive for the brain-wasting disease.

Japan is the largest overseas market for US beef. Agriculture Department officials and cattle industry executives tried to allay fears that American beef supplies had become infected, saying the US inspection system was working effectively.

The farm from where the cow originated was quarantined and officials were tracing the movement of the cow to the slaughterhouse, and the flow of the meat to three processing plants in Washington state.

"We remain confident in the safety of our food supply," Agriculture Secretary Ann Veneman said in announcing the first suspected US case of mad cow disease.

Veneman also assured Americans that no foul play was suspected.

Mad cow disease attacks the brains of cattle. It sprang up in Britain in 1986 and spread through countries in Europe and Asia, prompting massive destruction of herds and decimating the European beef industry.

The human form of mad cow disease so far has killed 143 people in Britain and ten elsewhere, none in the United States.

Veneman said the risk to human health in this US case was "extremely low."

Nonetheless, US beef producers worried that they could suffer heavily from a mad cow scare. Consumers Union, the publisher of Consumer Reports magazine, called on the government to test more cows for the disease, formally known as bovine spongiform encephalopathy.

"The US needs to be far more proactive in protecting the American food supply," said Michael Hansen, a senior research associate. "We are very concerned that the diseased animal made it into the food supply and that the processing plants could be contaminated."

And a Government adviser said rapid action is needed to restore public confidence in US beef stocks.

Sean Ricard, the former chief economist of the National Farmers' Union, predicted a short-term slide in the price of beef in America.

But he was optimistic that the discovery of the first suspected case of so-called "mad cow" disease in the US was not the start of an outbreak.

Mr Ricard, who now advises the Government on food and agriculture, told BBC Radio 4's Today programme: "When BSE was finally accepted by the Government here, that it could jump to humans, it was after ten years of denial. America has had ten years of rigorous testing.

"This is one case."