Researchers at Stemagen in the United States took cells from the company's chief executive, Dr Samuel Wood, and another employee to create five human embryos.

The scientists say they are the first to use human adult cells to create cloned embryos that advanced to the stage known as a blastocyst.

A blastocyst is an embryo about five days old and contains 50 to 200 cells.

This is the stage at which stem cells - capable of forming any of the body's 220 cell types, including blood, bone and nerve tissue - can be harvested for research or therapy, a process that destroys the embryo.

The work represents a step towards the creation of personalised embryonic stem cells that can be used for medical treatments, as well as a limitless supply of stem cells for future research. Theoretically, it could also be seen as a step towards creating babies that are genetic copies of other people.

'No other scientific group has documented the cloning of an adult human cell, much less been able to grow it to blastocyst stage,' said Dr Andrew French, Stemagen's chief scientific officer and lead author of the research.

A paper on Stemagen's work was published online on Thursday by the journal Stem Cells.

But it won faint praise from top figures in the field, who said it simply provided more evidence that humans could be cloned, like sheep or mice.

The research also failed to produce any stem cells, as the embryos were destroyed in testing the cloned DNA against that of the two cell donors. But Stemagen scientists say creating stem cells was not the point of the exercise - which left their peers feeling disappointed.

'It is very convincing; it is just not that big a leap,' said Dr George Daley, president of the International Society for Stem Cell Research and a stem cell scientist at Children's Hospital Boston, according to the Boston Globe paper.

'The real holy grail is to generate a pluripotent stem cell line from a cloned human blastocyst. It is only a matter of time before some group succeeds.'

Dr Wood said the company first wanted to prove it could clone an adult human cell, and was now turning to deriving cell lines.

'We have at least shown the opening to the cave that has the holy grail,' he said.

Cloning is done by removing the nucleus from an egg and replacing it with genetic material from another cell. In this case, the skin cells from the two men were placed inside eggs donated by young women at a fertility clinic that Dr Wood manages.

The egg is then induced into starting to divide as if it had been fertilised by sperm.

It is not clear whether the embryos would have been viable if implanted into a womb.

Dr Wood said Stemagen, which he started with a wealthy friend in 2005, was not interested in creating cloned babies. Rather, it wants to make stem cell lines for research and medical treatments.

Scientists envision that stem cells created from the clone of a patient - known as therapeutic cloning - could be turned into tissues such as brain cells for treating Parkinson's disease and pancreatic cells for treating diabetes.

NEW YORK TIMES