Led by Fred Gage, of the Salk Institute in San Diego, California, the researchers created the mice by injecting about 100,000 human embryonic stem cells per mouse into the brains of 14-day-old rodent foetuses.

Those mice were each born with about 0.1 per cent of human cells in each of their heads, a trace amount that the scientists say does not remotely come close to "humanising" the rodents.

"This illustrates that injecting human stem cells into mouse brains doesn't restructure the brain," Mr Gage said.

But the work adds to the growing ethical concerns of mixing human and animal cells when it comes to stem cell and cloning research. Mice are 97.5 per cent genetically identical to humans.

"The worry is if you humanise them too much you cross certain boundaries," said David Magnus, director of the Stanford Medical Centre for Biomedical Ethics. "But I don't think this research comes even close to that."

Researchers are nevertheless beginning to bump up against what bioethicists call the "yuck factor".

Three top cloning researchers provoked controversy with applications for a patent that contemplates fusing a complete set of human DNA into animal eggs in order to manufacturer human embryonic stem cells.

Doctors have transplanted pig valves into human hearts for years, and scientists have injected human cells into lab animals for even longer.

In 1995, Professor Charles Vacanti and Prof Linda Griffith-Cima caused a furore when he grew an engineered human ear on the back of a mouse.

But such experiments involving the brain pose an additional level of concern because some envision nightmare scenarios in which a human mind might be trapped in an animal head.

"Human diseases, such as Parkinson's disease, might be amenable to stem cell therapy, and it is conceivable, although unlikely, that an animal's cognitive abilities could also be affected by such therapy," said a report issued in April by the influential National Academies of Science, that sought to draw some ethical research boundaries.

The report recommended that such work be allowed, but with strict ethical guidelines established.

Mr Gage said the work published today in the journal Proceedings of the National Academy of Sciences is another step in overcoming one of the biggest technical hurdles confronting stem cell researchers: when exactly to inject the cells into patients.

The results suggest that human embryonic stem cells, once injected into people, will mature into the cells that surround them. No human is known to have received an injection of embryonic stem cells because so little is known about how those cells will mature once inside the body.