The country's President Roh Moo-Hyun congratulated the researchers for results he said were more akin to "magic" than science.
 
The cows have "antibodies specifically targeted against abnormal prions" known to cause bovine spongiform encephalopathy (BSE) - or mad cow disease - Seoul National University professor Hwang Woo-Sok said.

An abnormal prion is a diseased protein that triggers BSE and causes the destruction of brain tissue.

Beef contaminated with mad cow disease consumed by humans can cause a variant of Creutzfeldt-Jakob disease, which has claimed more than 140 lives in Europe. South Korea has reported no confirmed cases of mad cow disease.

In a ceremony attended by Roh, the professor called for joint work with Japanese scientists to obtain international approval of his achievement. He says he plans to send cloned calves from 15 pregnant cows to research facilities in Tsukuba, Japan.

Hwang's team also said they produced six "mini pigs" whose organs were more suitable for transplantation into humans, but the genetically modified pigs had died.

Roh lauded Hwang's achievement as a "precious" one that would help reduce fears of mad cow disease.

"You have achieved a very precious thing for human beings," he said.