By Michelle Roberts
BBC News, Health reporter in Lyon
Cloning sperm could enable men with very low sperm counts to become fathers, say US scientists.
Work is needed to ensure cloned sperm are healthy
By injecting a single healthy mouse sperm into a mouse egg from which the genetic material had been removed, they were able to make new sperm.
With some refining, the Cornell University team believes the technique could be used for infertile couples.
But experts at a fertility conference in Lyon raised safety concerns as mice made from cloned sperm were abnormal.
Professor Takumi Takeuchi and colleagues said four offspring had grown into "normal adults".
They conceded that much more work was needed to understand why some of the mouse embryos were abnormal and why others failed to develop.
But they said it had the potential to be a viable fertility treatment.
Fertility problems are extremely common, affecting one in seven couples attempting a first pregnancy. In about 40% of these cases, a problem with male fertility is a factor.
Where the man in a couple has problems making enough sperm, doctors are confronted with retrieving a single viable sperm to inject into an egg.
Professor Takeuchi said: "If you only have one healthy sperm you would be reluctant to use it for anything but fertilisation.
"But with this technique it should be possible to create enough sperm to be sure that the embryo which is implanted is healthy."
But Professor Keith Campbell, an expert on artificial gametes at the University of Nottingham in the UK, had reservations about the technology.
"I do not think it has got utility at the moment, apart from in the research lab. Work like this is still in its infancy and we have a lot to learn."
He said it would be important to look at future generations of mice born from the clone-made rodents because there might be abnormalities, such as a propensity to heart disease or diabetes, that could be passed down to offspring.
Proposed new UK laws would ban the use of artificially created sperm, and eggs, in fertility treatments.