Stem cells could soon be used to treat infertility, scientists have said.

Scientists have used embryonic stem cells to create the precursors of human sperm and eggs in an experiment they hope will aid infertility treatments.

Researchers in the US treated cells derived from early human embryos with proteins known to stimulate the formation of "germ cells", believing that watching the reproductive cells develop in the laboratory will help solve some of the mysteries of infertility.

A technique called RNA silencing was then used to control the activity of key genes involved in germ cell development.

The effect of the genes differed according to whether the cells were obtained from a male or female embryo.

Some male germ cells developed most of the way to becoming sperm cells.

Efforts to study infertility have been hampered by the fact that the human reproductive cycle cannot be properly studied in animals.

Professor Renee Reijo Pera, from the University of Stanford in California, one of the senior authors of the research reported online today in the journal Nature, said: "Ten to 15 percent of couples are infertile.

"About half of these cases are due to an inability to make eggs or sperm, and yet deleting or increasing the expression of genes in the womb to understand why is both impossible and unethical.

"Figuring out the genetic 'recipe' needed to develop human germ cells in the laboratory will give us the tools we need to trace what's going wrong for these people."