Scientists have identified three mutations that, if they occurred at the same time in nature, could turn a strain of bird flu now circulating in China into a potential pandemic virus that could spread among people.

The flu strain, known as H7N9, now mostly infects birds but it has infected at least 779 people in outbreaks in and around China, mainly related to poultry markets.

The World Health Organization said earlier this year that all bird flu viruses need constant monitoring, warning that their constantly changing nature makes them "a persistent and significant threat to public health."