By Jonathan Amos
BBC News Online science staff, in Seattle
South Korean scientists have cloned 30 human embryos to obtain cells they hope could one day be used to treat disease.
Seoul National University's Woo Suk Hwang, and colleagues, took the genetic material from normal cells in women donors and combined it with their eggs.
The resulting embryos were then grown up to produce so-called stem cells that can divide into any tissue in the body.
The aim is to use the cells to replace ones that have failed in patients with problems such as Alzheimer's disease.
"Because these cells carry the nuclear genome of the individual, after differentiation they could be expected to be transplanted without immune rejection for treatment of degenerative disorders," said Professor Hwang.
"Our approach opens the door for the use of these specially developed cells in transplantation medicine."
Details of the research are published online by the journal Science, on its Science Express website, and are to be discussed here on Thursday at the American Association for the Advancement of Science's annual meeting.
There have been claims in the past for the creation of human embryo clones to study stem cells - many of them disputed.
But no group has reported producing so many early-stage clones and seen their development progress to such an advanced stage.
The work has also been subjected to the rigorous scrutiny of independent scientists before publication in a major journal.
"These are the most advanced human embryo clones yet produced," Professor Hwang told BBC News Online.
The team says it sought approval for its work from an ethical review board and obtained informed consent from its women donors before proceeding with the work.
The team tells Science Express how it used 242 eggs in its experiments taken from 16 women.
In each case, material was transferred from the nucleus of a non-reproductive (somatic) cell, containing the woman's genetic blueprint, into a nucleus-free egg from the same donor.
Following this transfer, factors within the host egg's exterior, or cytoplasm, are believed to have reprogrammed the new nuclear contents by activating versatile embryonic genes, while silencing the more limited adult somatic cell genes.
In total, 30 embryos - exact genetic copies of their female donors - were then cultured to the so-called blastocyst stage at which stem cells could be extracted.
These special cells were seen to divide into all three of the main tissue types found in the human body, the researchers report.
The cells were even transplanted into mice to show they could differentiate into still more specific cell types, offering further proof of their "pluripotency".
The stated intention is to study human embryonic stem cells to see how they could be used as a therapeutic tool to treat disorders, such as diabetes, osteoarthritis, and
Parkinson's disease, among others, in which tissues in the body have begun to fail.
Editor-in-chief of the journal Science, Donald Kennedy, said: "The potential for embryonic stem cells is enormous, but researchers still must overcome significant scientific hurdles."
Professor Hwang said the embryos were the most advanced clones yet
And he added: "These results seem promising. But, it's important to remember that cell and tissue transplantation and gene therapy are still emerging technologies, and it may be years yet before embryonic stem cells can be used in transplantation medicine."
Addressing ethical concerns, he also called for a worldwide ban on activities which would seek to use this technology to create living children.
Professor Hwang, whose expertise has been developed in animal cloning, said any attempt to produce a baby would be "crazy".
"We will never try to produce cloned human beings," he said.
"During animal cloning, we experienced so many difficulties and dangers with deformities, especially in the internal organs."
Commenting on the Korean work, Roger Pedersen, professor of regenerative medicine, at the University of Cambridge, UK, told BBC News Online: "The present work has substantially advanced the cause of generating transplantable tissues that exactly match the patient's own immune system.
"These researchers' findings also make it possible to learn how to reprogramme the human genome to an embryonic state.
"This will likely accelerate the development of alternative ways of reprogramming human cells, which could in the future diminish the need to use human eggs for this purpose."