Cloning experts in Britain, however, cast doubt on the claims, saying that the research failed to show that the embryo was a genuine human clone created from the fusion of a human egg and an adult cell.
Professor Lee Bo-yeon, a medical researcher at the Kyunghee University Hospital in Seoul, told journalists that he had produced an embryo from an egg cell which had divided twice to the four-cell stage.
He said that he used a technique developed at the University of Hawaii in Honolulu, where scientists announced earlier this year that they had successfully cloned several generations of mice from a single female mouse. "Our experiment marked the first time that the more reliable cloning technology has been applied to human cells and might make human cloning more feasible," Professor Lee said.
The experiment, which has not been published in a peer-reviewed journal, was stopped when the embryo was at the four-cell stage because of a resolution adopted in 1993 by South Korean scientists prohibiting the research from going further.
"If implanted into the uterine wall of the carrier, we can assume that a human child would be formed and that it would have the same gene characteristics as that of the donor," Professor Lee said.
The Korean scientists said they removed the nucleus of an egg cell donated by a 30-year-old woman and replaced it with a nucleus taken from one of her ordinary "somatic" cells, which triggered the resulting embryo to divide twice to the four-cell stage.
However, experts from the Roslin Institute near Edinburgh, where Dolly the sheep was cloned in a similar process two years ago, said the South Korean work failed to show that the embryo was genuine and viable.
"Firstly, we do not believe the Korean group has sufficient scientific evidence to back itsclaim of having cloned a human embryo," said Dr Harry Griffin, the director of science at the institute. "The experiment was stopped when the embryo was seen dividing into four cells. A fertilised egg goes through the first few cell divisions - three in human - on `autopilot' and it is only after the eight-cell stage that the nucleus of the cells take control of further development of the embryo."
A key factor in showing that the experiment has produced a genuine, viable clone was to determine whether the egg cell's new nucleus really was in control of the cell. "By stopping the experiment when they did, the Korean researchers are unable to provide any evidence that the transferred nucleus had been successfully `reprogrammed'," Dr Griffin added.
The institute also cast doubt on the cloning credentials of the Korean team. "There is little in the reports to suggest that their work is part of a substantial programme of research," Dr Griffin said.
Professor Lee said the purpose of the research was to investigate ways of producing human embryos to generate tissue for transplant surgery, and not to create a cloned baby.
Donald Bruce, a biomedical ethicist and director of the Church of Scotland's Society Religion and Technology Project, said the Korean experiment had broken new ground, even if it is proved not to be scientifically valid. "The fact that someone tried to create cloned human embryos raises ethical concerns. Is it ethical to create a cloned embryo that you then have to destroy because it would be unethical to allow it to become a baby?" he said.
Animal Experiments that Could Apply to People
John Gurdon, then a young developmental biologist at Oxford University, showed that is it possible to clone a vertebrate animal. He took cells from the intestine of a South African frog and transferred them into unfertilised eggs to produce fully mature adults. Several attempts to repeat the work on mammals failed, leading some scientists to suggest it was impossible.
In the late Seventies, Steen Willadsen, a scientist working at the Agricultural Research Council near Cambridge, showed cloning was possible in higher animals, such as cattle, by manipulating embryo cells. By the early Eighties it was an established method of making clones but scientists still believed it was impossible to clone embryos from adult animals' cells.
The Roslin Institute, near Edinburgh, used nuclear transfer to clone sheep. A pair of sheep clones - Megan and Morag - developed by transferring embryonic cells into unfertilised eggs, were born in 1996. They were followed by Dolly, the first clone of an adult animal created by transferring a nucleus of an udder cell taken from a six-year-old ewe into an egg cell.
Scientists from the University of Hawaii in Honolulu successfully repeated the Dolly work using a slightly different approach. They managed to clone several generations of mice from a single adult female, using a process that was far less wasteful in terms of creating non-viable embryos. It was the first hard evidence to suggest that cloning could be applied to humans.Reuse content