The South Korean scientists who successfully cloned human embryos to extract stem cells have called for a global ban on cloning to make babies.
The embryos have been created in a process called "therapeutic cloning".
Woo Suk Hwang, who led the research, said reproductive cloning was clearly wrong, and should be outlawed.
He was speaking at a scientific convention in Seattle.
The researchers have shown they can produce stem cells from human embryos, in a procedure which is known as "therapeutic cloning."
The process means that stem cells can be grown into any cell in the body to treat various diseases.
The South Korean scientists are the latest group of mainstream scientists to call for a global ban, an idea which was first discussed at the United Nations three years ago.
Late last year, around 60 national academies of science from around the world called for a global ban on reproductive cloning, but said researchers should be free to experiment, as the Korean team did, with therapeutic cloning.
That was the position adopted by a number of governments led by France, Germany and Belgium during UN discussions.
But they are opposed by another block led by the United States and Spain which wants a complete ban on all cloning research.
In 2002, faced with this divide, the UN postponed negotiations for a year.
Last year, they reconvened and again postponed - this time until 2005.
The South Korean research illustrates the dilemma facing the UN.
It is aimed at therapeutic cloning, but the techniques it uses would also be used by people trying to create cloned babies.