Fourteen babies have been fathered by "sterile" men using a technique that injects eggs with very immature sperm cells.

Round spermatid injection (Rosi) has been tried before but dismissed as being of no clinical value because of its poor success rate.

In the new study, a team of Japanese scientists succeeded in delivering 14 children to 12 men and their partners by means of Rosi.

Spermatids are early-stage round-shaped sperm that can be found in the testes of men who would normally be considered sterile and advised to consider using a sperm donor to have children.

The successful births followed the injection of 76 electrically-stimulated eggs with spermatids taken from men aged 22 to 46.

None of the children, born between September 2011 and March 2014, have shown any sign of physical or mental problems, or genetic disorders, said the scientists, writing in the journal Proceedings of the National Academy of Sciences.

The team, led by Dr Atsushi Tanaka, from the Institute for ART (Assisted Reproductive Technology) in Fukuoka, Japan, concluded: "Round spermatid injection was effectively used in our clinic and resulted in the birth of 14 healthy babies.

"Although the current success rate of round spermatid injection is not very high compared with intra-cytoplasmic sperm injection (Icsi), this procedure can be the last resort for men who cannot produce spermatozoa but wish to use their own genetic material to produce offspring."

The scientists found that about 30% of infertile men lacking fully formed sperm or immature sperm at a late stage of development possessed round spermatids.

Previous attempts at Rosi may have failed because of difficulty identifying the cells and incomplete activation of recipient eggs, they said.

They added: "Computer-assisted identification of live spermatids and other types of spermatogenetic cells may become available in the near future."

The scientists acknowledged it might be premature to assume that Rosi is as safe as other forms of In-Vitro Fertilisation or Icsi.