The biotech company involved, Advanced Cell Technology (ACT) from Massachusetts, said it recognised the sensitivities involved and wanted a debate before pursing the work further.

Professor of Bioethics at Pennsylvania Glenn McGee discusses the claim

ACT said it had no intention of trying to create a human clone. Instead, it wants to try to grow organs and tissues in the lab for use in transplantation therapy.

"We have absolutely no interest or intent ever of cloning a human being," Michael West, President and CEO of ACT, said. "That's not the point at all. The point is to address transplantation."

In a technique similar to that used to produce Dolly the Sheep, the hybrid cell is made by removing the nucleus of a cow egg and inserting a human cell's nucleus. Only a small amount of the cow's genetic material remains.

The hybrids are said to have developed into a "master" state known as stem cells, which are capable of growing into any of the bodies many different types of tissues.

Transplant therapy

If this process can be controlled, ACT believe specific tissues could then be cultured for use in transplant therapy.

The announcement comes just a week after two other US groups announced that they had isolated and maintained in the lab embryonic stem cells taken from embryos and aborted foetal tissue.

The same biotech firm has cloned cows

However, the use of the nuclear transfer cloning technique on human cells will generate even greater controversy.

Several scientists have initially declared scepticism about what has been achieved because the research has yet to be subjected to peer review.

Dr John Gearhart of Johns Hopkins University, who was involved in one of the other studies, said: "It's not that I don't believe this biologically. I just think they could have given a little bit more assurance as to what was done here."