WASHINGTON (Reuters) - South Korean scientists described on Thursday how they cloned several human embryos and extracted valuable stem cells from one, and said their achievement showed an immediate need for a global ban on cloning to make babies.

They are the first researchers to prove they cloned a human being and said they did it not to make a baby but for the purposes of therapeutic cloning.

It could eventually involve taking a plug of skin from a patient and using it to grow perfectly matched tissue or even organs to treat diseases ranging from diabetes to Alzheimer's.

Woo Suk Hwang of Seoul National University, who led the study, and his colleagues said it was clearly wrong to use the technique for making an embryo that would be put into a woman's womb to grow into a baby.

"We call for a ban on reproductive cloning," Shin Yong Moon of Seoul National University, director of the center where the research was done, told a news conference in Seattle.

"To prevent reproductive cloning we would like to ask every country or every nation to have a law to prohibit reproductive cloning," added Moon, whose team's work was featured at the annual meeting of the American Association for the Advancement of Science.

President Bush opposes all forms of cloning and his administration has pressed for bans in Congress and in the United Nations, without success. Supporters of therapeutic cloning say the battle has left the entire field unregulated and allowed renegade scientists a legal opening to try to clone a human baby.

Ethicist Laurie Zoloth of Northwestern University said the Korean report showed it was time for lawmakers around the world to agree on what to do about cloning. "No one religion, no one moral authority, can claim to be the final arbiter of this work," she told the news conference.

LONG-RANGE POTENTIAL

Scientists welcomed the work as a breakthrough but stressed it would be years before any patient benefited from the technique.

"I emphasize that it is long-range, not short-range promise," Dr. Donald Kennedy, editor of the AAAS journal Science, which published the report, told the news conference.

"It represents, I think, an extraordinary series of technological accomplishments."

Disease researchers were also cautious in their welcome.

"If it turns out be true, it's a nice step forward," said Dr. Bob Goldstein of the Juvenile Diabetes Foundation. "It's measured skepticism only in the sense that, until these things are repeated (by other scientists), it always makes us nervous."

Opponents condemned the report.

"Cloning research is impossible to do without exploiting women. It should be banned immediately," said Daniel McConchie, a spokesman for the Chicago-based Center for Bioethics and Human Dignity.

"Cloning human beings is wrong. It is unethical to tinker with human life," said U.S. Rep. Joe Pitts, a Pennsylvania Republican who supports efforts to ban the technique.

Hwang's team created several clones using eggs and cumulus cells donated by Korean women who had independently approached them.

Cumulus cells are found in the ovaries and have been found to work especially well in cloning experiments.

The researchers removed the nuclei from the egg cells and replaced them with nuclei from the cumulus cells -- matching each woman's egg cell with her own cumulus cell. The nucleus contains 99 percent of a person's DNA.

Then they used a chemical trigger to start the eggs growing as if they had been fertilized by sperm.

HARD TO DO

Hwang stressed the difficulty of the experiment. Out of more than 200 tries, they got only 30 blastocysts -- the hollow balls of 100 to 200 cells that can be used as the source of stem cells.

When they tried to clone men using a piece of skin from the ear, they failed. They also failed when they tried to clone one woman using the hollowed-out egg of another woman.

Stem cells are found throughout the body and are a kind of master cell. Adult stem cells are difficult to find and to work with.

Kansas Republican Sen. Sam Brownback, who has led efforts to ban all human cloning, said using adult stem cells was the only acceptable route.

"I continue to be encouraged by the advances being made in adult stem cell treatments," he said in a statement. "We should all support the true scientific progress that is currently being made in the non-controversial fields of adult and non-embryonic stem cell research."