A medical team's quest to edit the genes of babies has been stopped by the Chinese government.
He Jiankui released five videos on Monday, saying he had used a gene-editing technology to alter the DNA of embryos for seven couples during fertility treatments, with one woman giving birth this month.
Mr He, a researcher at the Southern University of Science and Technology in the city of Shenzhen, said the woman had given birth to twin girls, known as Lulu and Nana.
The girls had their genes edited so they could resist being infected with HIV, he said.
The claim could not be independently verified and was met with a combination of scepticism and concern.
China's vice minister of science and technology, Xu Nanping, told state broadcaster CCTV that his ministry was strongly opposed to the work.
He described it as illegal and unacceptable.
An investigation has been ordered, he added.
The ministry was also quoted by state news agency Xinhua as saying: "The nature of this incident is extremely nasty, and relevant bodies have been ordered to temporarily halt the scientific research activities of relevant personnel."
Mr He had made the claims at the Second International Summit on Human Genome Editing being held in Hong Kong.
Organisers of the conference said in a statement: "Even if the modifications are verified, the procedure was irresponsible and failed to conform with international norms."
The presidents of the US National Academy of Sciences and the US National Academy of Medicine said the work "clearly demonstrates the need for us to develop more specific standards and principles that can be agreed upon by the international scientific community".
Editing DNA is highly controversial and is only allowed in the US and UK in laboratory research.
In September 2017, the practice of altering human embryos' blueprint for life was successfully carried out in the UK for the first time.
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Scientists at the Francis Crick Institute in London modified 41 embryos, donated by couples who no longer needed them for IVF, to help them explain what goes wrong in infertility.
Instead of altering a gene, they turned off a genetic instruction essential for early embryo development to see how that affected growth.