By Caroline Ryan
BBC News Online health staff in Berlin
Cloning creates potentially dangerous abnormalities in embryos, researchers have warned at a German conference.
Many questions remain about cloning
Scientists from Cornell University in New York, US, found cloned mice embryos had significant development problems.
Far fewer problems were seen in embryos fertilised using conventional methods of assisted reproduction, such as IVF.
The team told the European Fertility Conference in Berlin its study on rodents offered further proof that reproductive cloning was unsafe.
Sixty-eight mouse eggs were fertilised using conventional assisted reproduction techniques (ART) and cloning.
It was found far fewer of the cloned embryos reached the blastocyst stage at which embryos are three to five days old.
The researchers also observed unusual patterns of genetic development in the clones.
Call for a ban
Dr Takumi Takeuchi, who led the research, said: "We found significantly impaired development in the cloned embryos compared with those derived from more conventional ART techniques and this has made us more convinced that reproductive cloning is unsafe and should not be applied to humans."
Other experts said the findings added to the body of evidence which indicated reproductive cloning was too dangerous to pursue.
Dr Andre van Steirteghem, executive director of the European Society of
Human Embryology and Reproduction, told BBC News Online: "There is absolutely general agreement that reproductive cloning should be banned.
"It's clear from all the available experiments that it's much too dangerous."
The conference also heard from Dr Shin-Yong Moon, from the National University of Seoul in Korea.
Last year, he announced the production of the first stem cell line to have been derived from cloned human embryos.
He said work was continuing to improve the technique.
"Our first attempt was clearly not very efficient; we only managed to harvest one stem cell line.
"We do not know whether this was due to something going wrong in the development of the cloned embryo, or whether we need to make small variations in our experimental procedures.
But he added: "This will become clearer as we proceed."