But a Japanese team of researchers, led by Teruhiko Wakayama of the RIKEN research institute in Kobe, developed techniques to get around this problem, producing clones from mice which had been kept at -20 degrees for 16 years. No special chemicals had been added to the mice to cryopreserve them, a situation which would also apply to animals frozen in the wild, such as mammoths trapped in the permafrost.

Dr Wakayama said this meant his team's success "offers a distinct chance to resurrect extinct animals or preserve endangered species". But he cautioned that before an extinct animal could be cloned, the difficulties of using eggs and a surrogate mother from a related living animal would have to be overcome. The dean of science at the University of NSW, Professor Mike Archer, said it was a "very exciting" development, if confirmed.

The results, which are published in the journal Proceedings Of The National Academy of Sciences, would give hope to other teams, including his own, who were convinced it would eventually be possible to clone extinct species, including the Tasmanian tiger, or thylacine, despite the extreme difficulties. Bringing back animals that humans had driven to extinction was an ethical issue, Professor Archer said. "We have a moral imperative to do that if the technology allows us."

But the director of the Australian Centre for Ancient DNA at the University of Adelaide, Alan Cooper, said he was sceptical that Dr Wakayama's team had achieved what they claimed. "It could be right, but I'm highly suspicious. It is very hard to believe the DNA would be intact enough." Professor Cooper said the Japanese team had had to use a lengthy process to get the cloned mice, which could have introduced contamination. Dr Wakayama's team took the DNA-containing nuclei from the brain cells of the frozen mice and injected them into empty mice eggs, to produce cloned embryos from which they extracted embryonic stem cells. They then used these cells and more eggs to produce cloned embryos that grew into mice in a surrogate mother.

In a separate development, Dr Wakayama's team also created live mice by injecting dead sperm into mice eggs.