Cloning human beings for the purpose of reproduction is medically unsafe and should be banned, a panel of the National Academy of Sciences concluded Friday.

The scientific report comes even as White House bioethics advisers are weighing the benefits of medical advances against the moral hazards of human cloning. On Thursday President Bush challenged the ethics group to be the "conscience of the country."

The academy's report said: "Human reproductive cloning should not now be practiced. It is dangerous and likely to fail."

Animal cloning has shown that "only a small percentage of attempts are successful; that many of the clones die during gestation, even in late stages; that newborn clones are often abnormal; and that the procedures may carry serious risks for the mother," said the panel on the scientific and medical aspects of human cloning.

However, the panel of scientists added that the ban should not extend to cloning of embryos in order to extract stem cells that have the potential to treat life-threatening diseases. That practice is sometimes called therapeutic cloning to differentiate it from reproductive cloning.

The science panel urged that the safety of reproductive cloning be re-evaluated every five years but that the procedure be banned during that time.

"The panel believes that no responsible scientists or physicians are likely to undertake to clone a human," the report said. "Nevertheless, no voluntary system that is established to restrict reproductive cloning is likely to be completely effective."

The panel focused only on medical and scientific issues, saying it was leaving to others any discussion of the ethical, religious or social questions surrounding cloning.

Across town, the President's Council on Bioethics was diving into the details of human cloning. There is considerable support in Congress to ban the cloning of a human being for purposes of creating another human, but lawmakers are divided on cloning cells for research and medical treatment.

Even as his advisers were deliberating, Bush repeated his opposition to all human cloning Thursday, but said the group can serve an important role in helping Americans understand the issue.

"I have spoken clearly on cloning. I just don't think it's right," Bush told the council, which met with him at the White House. "On the other hand, there is going to be a lot of nuance and subtlety to the issue, I presume. And I think this is very important for you all to help the nation understand what this means."

Bush created the council, a mix of ethicists, doctors, lawyers and philosophers, after wrestling with whether to allow federal funding for research that used stem cells derived from embryos. He said he hopes the group will help as he faces similar balancing acts in the future.

"I really think you can help be the conscience of the country," he said.

Bush said it would help people like him understand how to come to grips with how medicine and science interface with the dignity of life, "and the notion that ... there is a creator."

The 18-member council will examine stem cell research, as well as euthanasia and assisted reproduction, typically in vitro fertilization.

To date, no one has cloned a human, which would be the genetic equivalent of a twin brother or sister born later. But scientists have cloned several animals, and last fall, researchers announced they had created a human embryo clone to provide stem cells for research.

The House has already passed a ban on human cloning and the Senate may take up the issue as soon as this spring. Council chairman Leon Kass predicted the council may have a recommendation by summer.

"We are going to try and do a good job, rather than bend ourselves out of shape to influence the Senate debate," he said.

Many in the Senate favor a ban that excludes research and medical treatments that do not involve implanting a cloned embryo into a woman's uterus. For instance, researchers believe they could clone the cells of a patient in order to create embryonic stem cells that are less likely to be rejected when used in treating his or her disease.

Opponents argue that this sort of treatment would involve destroying the newly created embryo, which they say is a human life on its own, and could lead to more objectionable forms of cloning.

On Friday, the council was debating the merits of each argument.

By contrast, Thursday's discussion was highly theoretical. The council is dominated by academics, and at times its session resembled a graduate seminar.

Several council members pointed with outrage to advertisements in their college newspapers for egg donors that are specifically looking for women with certain test scores or physical characteristics.

"I'm just disgusted by this," said Robert P. George, a professor of jurisprudence at Princeton University. "It strikes me as quite dehumanizing."

But others wondered what ethical boundaries it crosses. "You certainly wouldn't take eggs from someone with a genetic disease," said Kass, an ethicist at the University of Chicago.