A Chinese scientist claims he has helped create the world’s first genetically edited babies.

In research that would be illegal if carried out in most countries, Professor He Jiankui and his team said they altered the embryos of seven couples undergoing fertility treatment.

One successful pregnancy resulting from this treatment resulted in twin girls born earlier this month that the scientists claimed were naturally resistant to HIV.

Scientists have lined up to criticise the research which, if it turns out to be true, would represent a profound leap of science and ethics.

A US scientist also confirmed that he took part in the work in China, but this kind of gene editing is banned in his home country because the DNA changes can pass to future generations and risk harming other genes.

Many mainstream scientists therefore think it is too unsafe to try, and some denounced the experiment as “monstrous” human experimentation.

Professor He said his goal was not to cure or prevent an inherited disease, but to try to bestow a trait that few people naturally have: an ability to resist possible future infection with HIV, the Aids virus.

He said the parents involved declined to be identified or interviewed, and he would not say where they live or where the work was done.

There is no independent confirmation of the claim, and it has not been published in a journal, where it would be vetted by other experts. He revealed it on Monday in Hong Kong to one of the organisers of an international conference on gene editing that is set to begin on Tuesday, and earlier in exclusive interviews with the Associated Press.

“I feel a strong responsibility that it’s not just to make a first, but also make it an example,” he told AP. “Society will decide what to do next” in terms of allowing or forbidding such science.

Professor Julian Savulescu, an ethicist at the University of Oxford, said considering the effective ways to prevent HIV in healthy individuals and to treat those suffering from the disease, the work undertaken in China was indefensible.

“If true, this experiment is monstrous,” he said.

“Gene editing itself is experimental and is still associated with off-target mutations, capable of causing genetic problems early and later in life, including the development of cancer.”

Dr Eric Topol, who heads the Scripps Research Translational Institute in California, agreed that the research was “far too premature” given the current status of gene editing research.

“We’re dealing with the operating instructions of a human being. It’s a big deal,” he said.

However, one famed geneticist, Harvard University’s Professor George Church, defended attempting gene editing for HIV, which he called “a major and growing public health threat”.

“I think this is justifiable,” Professor Church said of that goal.

In recent years scientists have discovered a relatively easy way to edit genes, the strands of DNA that govern the body, using a tool that makes it possible to supply a needed gene or disable one that’s causing problems.

It’s only recently been tried in adults to treat deadly diseases, and the changes are confined to that person. Editing sperm, eggs or embryos is different – the changes can be inherited.

Support free-thinking journalism and attend Independent events

While this is outlawed in the US and the UK, China outlaws human cloning but not specifically gene editing.

Profesor He studied at Rice and Stanford universities in the US before opening a lab at the Southern University of Science and Technology in Shenzhen, where he also has two genetics companies.

The US scientist who worked with him on this project after Professor He returned to China was physics and bioengineering professor Michael Deem, who was his adviser at Rice. Professor Deem also holds what he called “a small stake” in and serves on the scientific advisory boards of Professor He’s two companies.

The Chinese scientist said he chose to try embryo gene editing for HIV because those infections are a big problem in China. He sought to disable a gene called CCR5 that allows HIV to enter a cell.

All the men in the project had HIV and all the women did not, but the gene editing was not aimed at preventing the small risk of transmission, Professor He said. The fathers had their infections deeply suppressed by standard HIV medicines and there are simple ways to keep them from infecting offspring.

Instead, the appeal was to offer couples affected by HIV a chance to have a child that might be protected from a similar fate.

He said the gene editing occurred during IVF, or lab dish fertilisation. First, sperm was “washed” to separate it from semen, the fluid where HIV can lurk. A single sperm was placed into a single egg to create an embryo. Then the gene editing tool was used.

When the embryos were three to five days old, a few cells were removed and checked for editing. Couples could choose whether to use edited or unedited embryos for pregnancy attempts. Eleven embryos were used in six attempts before the twin pregnancy was achieved, Professor He said.

Tests suggest that one twin had both copies of the intended gene altered and the other twin had just one altered, with no evidence of harm to other genes, he said. People with one copy can still get HIV.

Several scientists reviewed materials that Professor He provided to AP and said tests so far are insufficient to draw conclusions.

It’s unclear whether participants fully understood the purpose and potential risks and benefits. For example, consent forms called the project an “Aids vaccine development” programme.

The study participants are not ethicists, Professor He said, but “are as much authorities on what is correct and what is wrong because it’s their life on the line”.

“I believe this is going to help the families and their children,” he said. If it causes unwanted side effects or harm, “I would feel the same pain as they do and it’s going to be my own responsibility”.

Other researchers said that whatever the ultimate conclusions, Professor He’s experiments should serve as a warning of what is to come.

“It was inevitable that someone trying to reach the limelight would likely try this. However it is often better to be safe than to be first,” said Professor Peter Braude, a reproductive health specialist at King’s College London.

Dr Yalda Jamshidi, a human genetics expert at St George’s, University of London, added: “Whether the results stand up to scrutiny or not we need as a society to think hard and fast about when and where we are willing to take the risks that come with any new therapeutic treatment, particularly ones that could affect future generations.”