Dr Francis Collins, National Institutes of Health director, was responding to reportsthat the first embryos had been modified in China. 

He argued there were "serious and unquantifiable safety issues", big ethical questions and no compelling medical reason to do it.

He said the NIH would not fund such research in the US.

The advent of "Crispr technology" - which is a more precise way of editing DNA than anything that has come before - has spurred huge progress in genetics.

But there had been growing concern these tremendous advances were making the modification of human embryos more likely.

Dr Tony Perry, a pioneer in cloning, told the BBC News website in January that designer babies were no longer "HG Wells" territory.

Concerns were also raised in the journal Nature as rumours circulated that it had already taken place.