Human cloning has been used to produce early embryos, marking a "significant step" for medicine, say US scientists.
The cloned embryos were used as a source of stem cells, which can make new heart muscle, bone, brain tissue or any other type of cell in the body.
The study, published in the journal Cell, used methods like those that produced Dolly the sheep in the UK.
However, researchers say other sources of stem cells may be easier, cheaper and less controversial.
Opponents say it is unethical to experiment on human embryos and have called for a ban.
A South Korean scientist, Hwang Woo-suk, did claim to have created stem cells from cloned human embryos, but was found to have faked the evidence.
Now a team at the Oregon Health and Science University have developed the embryo to the blastocyst stage - around 150 cells - which is enough to provide a source of embryonic stem cells.
Dr David King, from the campaign group Human Genetics Alert, warned that: "Scientists have finally delivered the baby that would-be human cloners have been waiting for: a method for reliably creating cloned human embryos.
"This makes it imperative that we create an international legal ban on human cloning before any more research like this takes place. It is irresponsible in the extreme to have published this research."
However advocates of the new technique say that the embryos created from this technique could never become viable human beings.
