Image caption The US Department of Agriculture has said the disease will not affect beef exports

Two of South Korea's biggest retailers have halted sales of US beef after the discovery of a case of "mad cow" disease in the US.

Lotte Mart and Home Plus have temporarily suspended sales at their stores after a dairy cow in California was found infected with the disease.

Meanwhile, South Korean authorities said they will step up checks on beef imports from the US.

South Korea imported 107,000 tonnes of beef from the US in 2011.

"We stopped sales from today," said Chung Won-hun, a spokesman for Lotte Mart which is the second-biggest supermarkets chain in South Korea.

"Not that there were any quality issues in the meat but because consumers were worried."

Home Plus, the third-largest South Korean retailer, is a subsidiary of UK firm Tesco.

"We are gathering all the information regarding the mad cow disease and are evaluating how to respond to the situation," an official at the ministry for food, agriculture, forestry and fisheries in Seoul told the BBC.

However, the official said the ministry had not yet decided on a suspension of US beef imports.

Media playback is unsupported on your device Media caption USDA chief veterinary officer John Clifford: "This particular animal did not enter the food supply"

"If we take any measure, it would be made public within a day," he said.

The case announced on Tuesday in California is the fourth in the US so far, and the first since 2006.

The US has tried to allay any fears about the safety of its exports.

John Clifford, the chief veterinary officer for the US Department of Agriculture, said that "there is really no cause for alarm here with regard to this animal".

The European Comission said on Wednesday no measures would be imposed on imports from the US and said it believed steps were being taken to keep the disease out of the food chain.

The disease's proper name is bovine spongiform encephalopathy (BSE) and its first outbreak hit the US in 2003. At that time, South Korea banned all imports of American beef, though it has since lifted the restrictions.

BSE is always fatal in cattle, and research suggests humans can contract a similar deadly brain disease, vCJD, by eating infected meat from infected animals.

The disease causes personality change, loss of body function, and eventually death.

Although infected meat is no longer widely viewed as a risk, there has been continuing concern in the UK over human-to-human transmission through blood transfusions.