US President George W Bush has said he will veto any legislation that would ease funding restrictions on embryo stem cell research.
I am against science that destroys life to save life: Bush
He was speaking a day after South Korean scientists announced they had made stem cells tailored to the individual for the first time.
"I'm very concerned about cloning," said Mr Bush. "I worry about a world in which cloning would be acceptable."
Next week, the US Congress is to discuss funding for such research.
The House of Representatives is to debate legislation to expand the number of stem cell lines that are eligible for federally-funded research.
Supporters of the bill believe the vote will be close.
President Bush said he was a strong supporter of adult stem cell research, but using material from human embryos was a different matter.
"I've made it very clear... the use of federal money, taxpayers' money, to promote science which destroys life in order to save life, I'm against that," he told reporters.
"If the bill does that, I will veto it," Mr Bush said, threatening to use his presidential right for the first time in almost five years that he has been in office.
South Korean scientists made headlines by announcing they had made 11 new stem cell lines by taking genetic material from the patient and putting it into a donated egg.
The resultant cells were a perfect match for the individual and could mean treatments for diseases like diabetes without problems of rejection.
STEM CELL MILESTONES
1960s: Research begins on stem cells taken from adult tissue
1968: Adult stem cells used to treat immunodeficient patient
1998: US scientists grow stem cells from human embryos and germ cells, establishing cell lines still in use today
2001: Embryonic stem cell turned into a blood cell
2004: South Korean scientists clone 30 human embryos and develop them over several days
2005: Korean team develops stem cells tailored to match individual patients
The study, published in Science, has been hailed as a major advance.
But experts warn that there is a risk the cells could become cancerous.
And the Korean team admits much work is needed before stem cell techniques can be perfected.
The stem cell lines produced by the Koreans from patients with disease will likely also display some of the characteristics of that disease.
Professor Chris Higgins, from the UK Medical Research Council, said: "It really is an advance. It offers the possibility of stem cell therapies without rejection.
But Julia Millington, of the ProLife Alliance in the UK, said: "Cloning for research purposes, which involves the manufacture of human embryos destined for experimentation and subsequent destruction, is profoundly unethical.
"Experimentation upon human life at any stage of development has no place in a civilised society."