The United States has reported
its first suspected case of "mad cow disease", or BSE, in Washington state.
The diseased cow is undergoing tests
Agriculture Secretary Ann Veneman said a Holstein cow had tested positive - but she insisted US beef was safe.
In response, a number of countries in Asia, including America's biggest importer, Japan, have moved swiftly to ban US beef.
BSE has been linked to new variant Creutzfeldt-Jakob Disease (vCJD), a human brain-wasting disease.
The diseased cow was tested for BSE on 9 December and a tissue sample was being flown by military jet to the UK for further tests.
The results will take a few days to come through.
BOVINE SPONGIFORM ENCEPHALOPATHY
First surfaced mid-1980s
Can pass to humans through infected beef products
Human form of disease called vCJD
vCJD has killed 137 people, mainly in the UK
The farm near Yakima, Washington, where the cow was found has been quarantined.
"We remain confident in the safety of our food supply," Ms Veneman told reporters - adding that she planned to serve beef on Christmas Day.
The dairy cow had been sick or injured and was never destined for the public food supply, Ms Veneman said.
In Tokyo, Japanese agriculture ministry spokesman Hiroaki Ogura said a ban on US beef products took effect immediately, until further information could be gathered.
South Korea - another key US market - followed suit, halting customs inspection of US beef and effectively blocking its entry into the country's markets.
The BBC's Michael Buchanan in Washington says while that will concern exporters, the real harm could come if American consumers stop eating beef and deal a potentially devastating blow to the $175bn US cattle industry.
Canada, the third biggest foreign market for US beef, said it would wait for confirmation on the test results before taking any action.
In August, the US eased a ban on Canadian beef imports after a single case of the disease was found at a farm in Alberta in May. The two cases do not appear to be connected.
COUNTRIES THAT HAVE BANNED US BEEF
A private study released in November estimated that Canada's beef industry lost $2.5bn (C$3.3bn) in the six months after its mad cow case was discovered.
First diagnosed in Britain in 1986, BSE (Bovine Spongiform Encephalopathy) affected 178,000 British cattle and resulted in the eventual destruction of 3.7 million animals.
It cost British farming billions of dollars as countries around the world banned British beef.
'Abundance of caution'
But Ms Veneman insisted that since 1990, steps had been taken in the US to ensure there could be no similar spread of the disease through American herds.
"Even though the risk to human health is minimal, we will take all appropriate actions out of an abundance of caution," she said.
TOP IMPORTERS OF US BEEF
4. South Korea
5. Hong Kong
Source: US Meat Export Federation
Correspondents say the US beef business is booming - partly due to the popularity of the protein-rich Atkins diet.
The agriculture minister said she was working with the Department of Homeland Security to assure the public that the suspected case was not terrorist related, or connected with the current terrorism alert in the US.
Most of the beef raised in the United States is consumed by the domestic market, with 10% exported to foreign markets, according to the US Department of Agriculture.
The most important foreign markets are Japan, Mexico, Canada, South Korea and Hong Kong, according to the US Meat Export Federation.