Scientists have finally succeeded in using cloning to create human embryonic stem cells, a step toward developing replacement tissue to treat diseases but one that might also hasten the day when it will be possible to create cloned babies.

The researchers, at Oregon Health and Science University, took skin cells from a baby with a genetic disease and fused them with donated human eggs to create human embryos that were genetically identical to the 8-month-old. They then extracted stem cells from those embryos.

The embryo-creation technique is essentially the same as that used to create Dolly the sheep and the many cloned animals that have followed. In those cases, the embryos were implanted in the wombs of surrogate mothers.

The Oregon researchers, led by Prof. Shoukhrat Mitalipov, did not implant their human embryos and said they had no intention of doing so. They say their technique, in any case, would not lead to the birth of a viable baby. The same technique, tried in monkeys for years, never resulted in the birth of a cloned monkey, they said.

Nonetheless, the fact that the scientists were able to get cloned human embryos to survive long enough for stem cell extraction is likely to be seen as a step on the way to human reproductive cloning.

The Conference of Catholic Bishops, for instance, said Wednesday that the research “will be taken up by those who want to produce cloned children as ‘copies’ of other people.”

Cardinal Sean O’Malley of Boston said human cloning was immoral, even if used for therapeutic purposes, because it “treats human being as products, manufactured to order to suit other people’s wishes.”

The Oregon researchers, who published a paper on their work in the journal Cell, say their goal is what has been called therapeutic cloning: making embryonic stem cells that are genetically identical to a particular patient.

Embryonic stem cells can turn into any type of cell in the body, like heart cells, muscles or neurons. That raises the hope that one day the cells will be turned into replacement tissue or even replacement organs to treat a host of diseases.

Human embryonic cells are now mainly derived from embryos created by fertilization in fertility clinics. But tissues created from those stem cells would not genetically match a patient, meaning steps might be needed to prevent rejection.

Scientists have been trying for more than 10 years to create human embryonic stem cells using the cloning method.

Still, the demand for therapeutic cloning may be less now than it was a decade ago because scientists can now use adult skin cells to create a stem cell very similar to embryonic cells, but without the need for embryos. These are called induced pluripotent stem cells. The induced cells also sidestep the ethical issues of embryonic stem cells, which are often created by destroying embryos.

Attempts to use either type of cell for therapy remain at the early stages of research, so it is not clear which will turn out to be better. So-called adult stem cells, taken from blood, fat or other parts of the body, are another possible option.

Dr. Mitalipov and his colleagues created monkey stem cells through cloning in 2007 and since then have been trying to tweak the technique to work with human cells.

A drawback of therapeutic cloning is that there might never be enough human eggs available to treat all patients, should the therapy ever work. Egg donors can suffer serious side effects from the powerful hormones needed to generate multiple eggs.

Dr. Mitalipov said the technique was efficient enough that one donation — which can include multiple eggs — would probably be enough to generate a stem cell line, even accounting for failures.

Most patients who would want replacement tissues are likely to be old. The researchers must still show they can produce stem cells starting with skin cells from adults.