The US team has developed a new method for handling the donor egg

A team in the US created dozens of cloned embryos from a 10-year-old male macaque, the journal Nature reports.

This could make it easier to clone human embryos for use in research.

It raises the prospect of developing transplant tissues to treat diseases such as diabetes and Parkinson's that will not be rejected by the body.

The American group was able to extract stem cells from some of the cloned monkey embryos, persuading them to develop into mature heart and nerve cells in the laboratory.

Other scientists have welcomed news of the advance. Robin Lovell-Badge, a UK stem cell scientist at the National Institute for Medical Research in Mill Hill, near London, said: "Although this work has not been published yet, it is potentially significant.

"There has been a worry that primates may prove to be difficult in terms of cloning."

Medical promise

This would have been a huge setback for researchers working to develop new medical therapies based on embryonic stem cells.

In cloning to obtain stem cells, DNA from an adult animal is inserted into an unfertilized egg that has had its own genetic material removed. The egg is then encouraged to grow into an early embryo, from which stem cells can be extracted.

STEM CELLS Stem cells are master cells, obtained from early-stage human embryos, with potential to develop into any of the body's tissue types There are different types, but scientists believe the most useful stem cells come from the tissue of embryos The copy pre-embryo created for therapeutic cloning is destroyed in the process

Because stem cells are the forerunners of all tissues in the body, scientists hope they might one day be able to use these progenitors to create transplant tissues that are genetically matched to patients with degenerative conditions - such as diabetes - without the fear of rejection by the body.

Human cloning has been dogged by technical difficulties and controversies over faked research.

In 2004, a South Korean team announced that it had created the first cloned human embryos and extracted stem cells from them. But the study was later retracted when it emerged the lead author, Dr Hwang Woo-suk, had fabricated his work.

The only other published example of a human embryonic clone was created at Newcastle University, UK. But the clones survived for only a few days and did not produce any stem cells.

Human cloning clues

The technique used to generate the cloned macaque embryos is called somatic cell nuclear transfer (SCNT). This is the same basic procedure used to create Dolly the sheep and other cloned mammals.

But the lead author of the latest study, Dr Shoukhrat Mitalipov, has pioneered a novel way of handling the donor eggs during the cloning process.

The macaque monkey is a standard animal in laboratory work

However, Dr Mitalipov and his colleagues believe both of these could damage primate eggs. Instead, they used an illumination technique which allowed the scientists to efficiently remove the cell's nucleus without resorting to the traditional approaches.

The new technique, called Oosight, uses polarised light to visualise microscopic cells in real time. The scientists found this resulted in a much improved survival rate for developing clones.

'Convincing evidence'

In a statement, Professor Alison Murdoch and Dr Mary Herbert, of the North-East England Stem Cell Institute (NESCI), said the work "provides the first convincing evidence that nuclear reprogramming is feasible in primates".

They added: "This is a very exciting development which takes us several steps closer to the production of patient-specific stem cells to treat life-limiting conditions such as Parkinson's, motor neurone disease, Huntington's disease and cystic fibrosis.

"By providing proof of principle in a primate model, Dr Mitalipov and his colleagues have made an important step towards realising the therapeutic potential of nuclear transfer in humans."

Professor Ian Wilmut, director of the Scottish Centre for Regenerative Medicine at the University of Edinburgh, commented: "Cloned cells produced with the genetic material of a patient who has inherited a disease would have the abnormalities associated with the disease."

The researcher, who led the project which resulted in Dolly the sheep, added: "The methods in this paper are a significant step towards this objective."

Implantation efforts

But the development was not welcomed by everybody.

Josephine Quintavalle, director of the campaign group Comment on Reproductive Ethics (Core) told BBC News: "Bringing a clone to term is the only way to show that the cloned tissue is safe."

Ms Quintavalle pointed out that clones were not the only potential source of embryonic stem cells, and that other options such as cord blood existed.

The scientists behind the latest work reportedly tried to implant about 100 cloned embryos into the wombs of around 50 surrogate female macaques. However, their efforts did not result in the birth of any offspring.

But one author of the study said this could be down to bad luck. For example, Dolly the sheep - the first clone of an adult mammal - was only created after 277 attempts.

Dr Mitalipov is affiliated to the Oregon National Primate Research Center and the Oregon Stem Cell Center.