The Stem Cell Research Enhancement Act, passed by the US Senate on Tuesday, would lift rules Bush set in 2001 that make federal funds available only for research on a small number of embryonic stem cell lines which existed at that time. Supporters have said the research offers major hope to cure many life-threatening illnesses including Parkinsons disease and Alzheimers' disease.

But the president had repeatedly threatened to veto the bill on moral grounds. "We must also remember that embryonic stem cells come from human embryos that are destroyed for their cells. Each of these human embryos is a unique human life with inherent dignity and matchless value," Bush said in his comments to specially invited families at the White House.

"Some people argue that finding new cures for disease requires the destruction of human embryos," Bush said, before adding: "I disagree. "I believe that with the right techniques and the right policies we can achieve scientific progress while living up to our ethical responsibilities."

Last week, the journal Science published a letter from three researchers criticizing the claim that adult stem cells are preferable to embryonic stem cells. The authors included Dr. Steven Teitelbaum of Washington University in St. Louis, who has used adult stem cells to treat bone diseases in children. The authors wrote that the exaggerated claims for adult stem cells "mislead laypeople and cruelly deceive patients."

The bill heading for Bush's desk would expand federal funding of work on stem cells taken from embryos. Such cells come from extra embryos originally created for in-vitro fertilization. Many experts believe embryonic stem cells could one day help regenerate damaged tissue for patients with conditions such as diabetes, spinal cord injury or Parkinson's disease, though embryonic cells have not yet been tested in humans.

Adult stem cells, which usually come from bone marrow transplants or umbilical cord blood, are widely considered less flexible than embryonic stem cells in forming many types of tissue. Yet adult stem cells already are in common use for certain conditions, such as replenishing immune cells after cancer treatment and treating some bone and blood disorders. Bush allowed limited funding of embryonic stem cell work in August 2001, but he banned funding of cells taken from embryos after that date. However, private foundations and companies have continued to fund new embryonic research. Many scientists and lawmakers argue that the federal funding limitation has hindered progress.

White House spokesman Ken Lisaius on Tuesday could not provide the name of a stem cell researcher who shares Rove's views on the superior promise of adult stem cells.