Scientists in China have for the first time used cloning techniques to create hybrid embryos that contain a mix of DNA from both humans and rabbits, according to a report in a scientific journal that has reignited the smoldering ethics debate over cloning research.

More than 100 of the hybrids, made by fusing human skin cells with rabbit eggs, were allowed to develop in laboratory dishes for several days before the scientists destroyed them to retrieve so-called embryonic stem cells from their interiors. Although scientists in Massachusetts had previously mixed human cells and cow eggs in a similar attempt to make hybrid embryos as a source of stem cells, those experiments were not successful.

Researchers said Wednesday they were hopeful that the rabbit work would lead to a new and plentiful source of embryonic stem cells for research and, eventually, for medical use. But theologians and others decried the work as unethical.

Some wondered aloud what, exactly, such a creature would be if it were transferred to a womb to develop to term.

The vast majority of the DNA in the embryos is human, with a small percentage of genetic material -- called mitochondrial DNA -- contributed by the rabbit egg. No one knows if such an embryo could develop into a viable fetus, though some experiments with other species suggest it would not.

The new work, led by Hui Zhen Sheng of Shanghai Second Medical University, appears in the latest issue of Cell Research, and was highlighted in a news report in the journal Nature. Cell Research is a peer-reviewed -- if little known in the United States -- bimonthly scientific journal affiliated with the Shanghai Institute of Cell Biology and the Chinese Academy of Sciences. Some researchers Wednesday said they were frustrated by the lack of details in the paper.

The team said they retrieved foreskin tissue from two five-year-old boys and two men, and facial tissue from a 60-year-old woman, as a source of skin cells. They fused those cells with New Zealand rabbit eggs from which the vast majority of rabbit DNA had been removed. More than 400 of those new, fused entities grew into early embryos, and more than 100 survived to the so-called blastocyst stage -- the point at which coveted stem cells begin to form.

The approach could help scientists wishing to mass-produce human embryos as sources of human embryonic stem cells. Stem cells can morph into all kinds of tissues and may be able to reverse the effects of various degenerative diseases. But to make cloned embryos, scientists need both normal body cells --

such as skin cells -- and egg cells, which have the unique capacity to "reprogram" the genes in body cells and make them behave as though they were embryo cells.

Since human egg cells are difficult and costly to retrieve from women's ovaries -- and because human egg retrieval poses certain risks to the donors --

scientists have been wanting to know whether animal eggs may serve as well. A major question has been whether the remnants of mitochondrial DNA that typically remain in an animal egg would be compatible with the human "nuclear DNA" contributed by the human cell.

The new work suggests that the answer to that question is "yes," scientists said -- though with a number of caveats. Most important, researchers said, the paper stops short of proving beyond a doubt that the stem cells retrieved from the hybrid embryos are truly capable of growing for long periods of time in lab dishes, and can turn into every known kind of cell.

But even so, said Douglas Melton, a Harvard cell biologist and cloning expert, the work is a big advance because it offers a new system for exploring the mechanisms by which egg cells get adult cells to act in embryonic ways.