More than 120 Chinese scientists on Monday condemned a researcher’s claim that he has – for the first time in history – edited the genes of two babies, in a case with profound medical and ethical implications.

Experimenting on humans is "insane" and carries grave potential consequences, said an open letter signed by scientists from several institutions, including Tsinghua, Peking and Fudan universities and the Chinese Academy of Sciences.

"These irreversible transformations of human genetic material, which are highly uncertain in science, will inevitably be mixed into the human gene pool," the letter said.

"Indeed, it’s possible that the children born this time are basically healthy for a period of time, but the potential risks and harm to the human group ... are immeasurable."

Researcher He Jiankui from the south-eastern city Shenzhen said the genes of two Chinese girls had been modified so they cannot be infected with HIV, according to a YouTube video posted Sunday. 

The girls, named Lulu and Nana, "came crying into the world as healthy as any other babies a few weeks ago," he said.

He claims he performed gene surgery on fertilized eggs using the CRISPR/Cas9 protein with instructions to prevent future HIV infection. He has yet to publish a paper about the highly controversial procedure. 

The Southern University of Science and Technology researcher said the gene surgery was successful and "no gene was changed except for the one to prevent HIV infection."

He is the founder of Direct Genomics, a private company that claims on its website to be "making genomics an affordable part of everyday patient care." He studied at Rice and Stanford universities in the United States.

Southern University of Science and Technology said in a statement Monday that He conducted the research outside the school and without the knowledge of the school or his department.

The university added He’s research violated academic ethics and norms and called for an official investigation into his work.

Shenzhen authorities announced Monday that a committee of medical ethics experts had been formed and is looking into He’s research. 

He said in the video he expected his work to be controversial but was willing to face the backlash on behalf of families who need gene-editing for "healing." 

"Enhancing IQ or selecting hair or eye colour ... should be banned," he added.

He could not be reached for further comment.

Peter Dabrock, chairman of the German Ethics Council, said Monday the work represents "irresponsible experiments on humans."

"Whether what Chinese researcher He claims is true, at present such attempts and even announcements are to be criticized in the strongest terms," Dabrock said, adding that secondary and late effects of gene editing are difficult to predict and control.  

The Chinese scientists also called for a comprehensive investigation into He’s research.

"Pandora's Box has been opened, and we may have a chance to close it before it is irreparable," they said.