For unclear reasons, the study's senior author, Gerald Schatten of the University of Pittsburgh, has asked his name be taken off the study, the journal Science reported.
The lead researcher, Hwang Woo Suk of Seoul National University, gained fame during the past two years with striking successes in cloning, The New York Times reported.
Last June, in the research involving Schatten, Hwang claimed establishing a line of human embryonic stem cells by transferring the nucleus of adult cells to a human egg, the nucleus of which had been removed. The project was similar to one he announced in 2004, but involved far fewer eggs.
The international group of scientists says questions about the validity of experiments are best resolved within the scientific community.
The letter was signed by Ian Wilmut of Edinburgh University, John Gearhart of Johns Hopkins University, Austin Smith of the University of Cambridge, Alan Trounson of Monash University, Keith Campbell of the University of Nottingham among others.