LONDON — Scientists hailed on Friday the announcement that British researchers have succeeded in creating the country's first — and the world's second — cloned embryo, a breakthrough that keeps Britain at the forefront of the fast-moving, potentially revolutionary field.

But some cautioned that the use of embryonic stem cells to treat serious illness could carry its own risk of exposure to disease.

A team of Newcastle University scientists, who last year were granted Britain's first license for human cloning, said Thursday they had successfully produced an early stage embryo cloned from a human cell using nuclear transfer.

Unusually, the team announced its progress to the media before publication of a peer-reviewed study. A paper has been submitted to Reproductive BioMedicine Online.

Reproductive BioMedicine Online said no date had been set for publishing a full, peer-reviewed study.

Nicola Mcintosh, spokeswoman at the Centre for Life at Newcastle University, said the announcement was made in response to publication of the South Korean team's work.

"We were asked to comment on the Korean announcement, and we anticipated that the media would ask where we were with our developments. So rather than wait a few days, we told them where we are," McIntosh said.

Britain, which four years ago became the world's first country to license cloning to create stem cells, joins South Korea on the leading edge of the research, which many scientists believe may lead to new treatments for a range of diseases.

A team of South Korean scientists last year became the first to clone a human embryo. On Thursday they announced they had dramatically sped up the creation of human embryonic stem cells, growing 11 new batches that for the first time were a genetic match for injured or sick patients.

The Newcastle researchers were granted a license in August by Britain's Human Fertilization and Embryology Authority. They hope eventually to create insulin-producing cells that could be transplanted into diabetics.

Two of the team, Alison Murdoch and Dr. Miodrag Stojkovic, said they were "delighted" by the Koreans' progress.

"They have shown conclusively that these techniques can be successful in humans," they said. "The promise of new treatments based on stem cell technology is moving nearer to becoming a realistic possibility."

The researchers are not using cloning to make babies. Instead, scientists create test-tube embryos to supply stem cells, the building blocks which give rise to every tissue in the body and which are a genetic match for a particular patient, preventing rejection by the immune system.

If scientists could harness the regenerative power of those stem cells, they might be able to repair damage from spinal cord injuries, diabetes, Parkinson's and other diseases.

Scientists caution that any therapy is still years away from being tested on people.

And experts writing in the British Medical Journal warned that stem cell therapy could put patients at risk of illnesses such as the human form of "mad cow" disease.

"The premature use of cell therapy could put many patients at risk of viral or prion diseases unless systems are in place for the appropriate selection and screening of donors for quality assurance," the scientists from King's College London and the National Blood Service wrote.

"Expansion of stem cell cultures could allow a single stem cell line to be used for many hundreds, if not thousands of patients, exponentially amplifying the potential risk of disease transmission from a single infected donor," they said.

Britons have been broadly supportive of stem cell research, although anti-abortion groups condemn it. Julia Millington of the ProLife Alliance said the Newcastle research was "profoundly unethical."

Reproductive cloning is banned by law in Britain, and a breach can result in a 10-year prison sentence. But some fear that advances in therapeutic cloning could also assist mavericks working toward the creation of cloned babies.

Oxford University ethicist Julian Savulescu argued, however, that it would be immoral not to pursue stem cell research.

"By holding back cloning research we may be responsible for the deaths of many people," he said. "It is now thoroughly immoral and irresponsible not to do more with this life-saving research."