Stemagen Corp. in La Jolla, California, destroyed the embryos while testing to make sure they were true clones. But the researchers, based at a fertility centre, said they believed their ready source of new human eggs would make their venture a success.
Other experts were sceptical about the claims, published in the journal Stem Cells. If verified, the team would be the first to prove they have cloned human beings as a source of stem cells, the master cells of the body.
There are several types of stem cells. Embryonic stem cells, made from days-old embryos, are considered the most powerful because they can give rise to all the cell types in the body.
The Stemagen team said they got five human embryos using skin cells from two adult men who work at the IVF centre. They said they had painstakingly verified that the embryos were clones of the two men.
"We hope it is a bit of a turning point for many more studies," Andrew French, who led the research, said.
They used a technique called somatic cell nuclear transfer, or SCNT, which involves hollowing out an egg cell and injecting the nucleus of a cell from the donor to be copied - in this case, the skin cells from the men.
It is the same technique used to make Dolly the sheep in 1996, the first mammal to be cloned from an adult. Researchers hope to use the technique to create tailor-made transplants of cells, tissue or organs for patients, treating injuries and diseases like juvenile diabetes.
"Since a significant percentage of couples undergoing fertility treatments appear willing to participate in this type of research, we believe the method described to obtain donated oocytes is a viable and ethically acceptable strategy," the researchers wrote.
Most successful so far
Some cloning experts said the work appeared to be genuine.
"This is the most successful description so far of the use of the cloning techniques with purely human material. However, it is still a long way from achieving the goal of obtaining embryonic stem cells," said Robin Lovell-Badge of Britain's Medical Research Council's division of stem cell biology.
"I hope that the authors have the opportunity to continue their work and derive embryo stem cell lines," Ian Wilmut, who led the team that cloned Dolly and who is now at the University of Edinburgh, said in an e-mail.
US President George W. Bush opposes the use of human embryos to make stem cells and has vetoed bills from Congress that would expand federal funding of this research.
South Korean scientist Hwang Woo-suk made headlines when he was found to have faked key parts of a report that his team had used cloning technology to make human embryos in 2004.
"We need to be ultra-cautious after the Hwang scandal and not make the same mistake all over again," said Dr Robert Lanza of Advanced Cell Technology, a Massachusetts company that is also trying to make human embryonic stem cells. "I'd really like to believe it, but I'm not sold yet," Lanza said.
Other teams have made stem cells they believe are similar to embryonic cells using a variety of techniques, including reprogramming a human egg cell alone, reprogramming ordinary skin cells into what are called induced pluripotent stem cells, or by taking one cell from a human embryo without harming the embryo.
But most stem cell experts agree it is important to continue trying to make stem cells from embryos too. - (Maggie Fox/Reuters Health)