A group of US scientists and activists have called for a global ban on the genetic modification of human embryos, warning the technology could have an irreversible impact on humanity.

The Centre for Genetics and Society (CGS) and the activist group Friends of the Earth issued the report a day before a major international meeting in Washington to discuss the ethical and policy issues surrounding the technology.

A technological innovation that can strategically edit out specific stretches of DNA could ultimately lead to the genetic modification of children, and should be halted before it starts being used, they argued.

"Like so many powerful new technologies, gene editing holds potential for both great benefit and great harm," an open letter published by the groups said.

"The implementation of heritable human genetic modification — often referred to as the creation of 'genetically modified humans' or 'designer babies' — could irrevocably alter the nature of the human species and society.

"Gene editing may hold some promise for somatic gene therapy (aimed at treating impaired tissues in a fully formed person).

"However, there is no medical justification for modifying human embryos or gametes in an effort to alter the genes of a future child."

Consulting researcher with CGS and author of the report Pete Shanks said although the technology was "recently the stuff of science fiction", the fantasy could become reality.

"Avoiding illness, which we can increasingly do, is very different than manufacturing to order," he said.

"Kids are unique and should be celebrated for who they are.

"Parenting is challenging — in a good way. It's about helping your kids be themselves, not some fantasy you invented.

"Once the process [of gene modification] begins, there will be no going back. This is a line we must not cross."

CGS executive director Marcy Darnovsky said engineering the genes we pass on to our children and future generations would be "highly risky, medically unnecessary, and socially fraught".

"The worst-case scenarios are pretty horrific: a genetics arms race between nations or within societies, a world in which affluent parents purchase the latest set of upgrades for their offspring, leading to the emergence of genetic 'haves' and 'have nots' ... a world with new forms of inequality, discrimination and conflict," she said.

Technology allows scientists to 'find and replace' genes

Advocates say the technology, CRISPR/Cas9, is the first step in allowing scientists to prevent heritable diseases.

Opponents worry about unknown effects on future generations and the temptation for future parents to pay for genetic enhancements such as greater intelligence or athletic ability.

CRISPR/Cas9 allows scientists to manipulate genes like the "find and replace" function in word processing changes text. Scientists introduce enzymes that bind to a mutated gene, such as one associated with disease, and then replace or repair it.

The technique, if used to alter the DNA of human sperm, eggs, or embryos, holds the promise of eliminating a host of inherited diseases.

But many scientists worry this could produce unknown effects on future generations, since the changes are passed on to offspring.

Scientists note that the gene-editing techniques can also be used to alter the DNA of non-reproductive cells to repair diseased genes. The objections come to so-called "germline editing" in which reproductive cells are modified.

In May, the White House endorsed a ban on germline editing pending further study of the ethical issues.

The latest report was released on the eve of a summit convened by the US National Academy of Sciences and its counterparts from China and the United Kingdom, after a series of events this year that have brought new urgency to the debate.

In March, a group of scientists led by one key developer of the CRISPR technique called for a voluntary research ban on the use of the technology for germline editing, reflecting fears about safety and eugenics.

Shortly after that, a team of Chinese scientists reported carrying out the first experiment to alter the DNA of human embryos.

That news ignited an outcry from some scientists, though others defended the Chinese research as careful and safe since it used only non-viable human embryos.

ABC/Reuters