Scientists have developed an improved gene editing tool that significantly reduces potentially dangerous "off-target" edits, promising an even more precise and efficient system for manipulating human DNA.

Editing the genes of living organisms, including humans, holds out great promise for treating diseases. But it could also be used to create "designer babies", prompting critics to call for a global ban on genetic modification of human embryos.

Today's news that US researchers have re-engineered the so-called CRISPR-Cas9 system to cut editing errors comes as experts meet in Washington for a three-day summit to discuss the ethical and policy issues surrounding the field.

The summit has been jointly organised by the academies of science in China, the UK and the US.

International experts are discussing the scientific, medical, ethical and governance issues surrounding advances in human gene editing research at the summit.

The new system allows scientists to edit genes by using biological "scissors" that operate a bit like a word-processing programme that can find and replace defects.

The approach has excited academic researchers and drug companies alike, since it offers a way to rewrite the DNA of diseased cells, and the technology has been quickly put to work in laboratories around the world.

While CRISPR is highly effective and relatively simple to use, one major shortcoming is that it can cut additional sites on the genome that are not targeted, potentially causing undesired genetic effects. These could include cancer.

Now researchers at the Broad Institute of MIT and Harvard and the McGovern Institute for Brain Research at MIT believe they have found a way round the problem by tweaking three amino acids to reduce off-target editing cuts.

Feng Zhang of the Broad Institute and colleagues said they were making the new, improved system immediately available to researchers worldwide. Their findings were published online in a paper in the journal Science.

CRISPR has been put to work in a range of fields, including crop breeding and engineering mosquitoes that cannot spread malaria. But major ethical concerns were sparked in April when a team in China published details of an experiment to alter the DNA of human embryos.

While gene editing could, in theory, be used to stop a range of life-threatening genetic diseases being passed on to future generations, such "germ line" modification would take medicine into a whole new area.

Opponents worry about unknown effects on future generations and the temptation for future parents to pay for genetic enhancements.

The question of how the revolutionary technology should be used is being tackled at the international summit in Washington, convened by the US National Academy of Sciences, alongside Britain's Royal Society and the Chinese Academy of Sciences.