By Kate Kelland, Health and Science Correspondent

LONDON, June 15 (Reuters) - 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".

At the moment, the H7N9 virus does not have the capability to spread sustainably from person to person. But scientists are worried it could at any time mutate into a form that does.

To assess this risk, researchers led by James Paulson of the Scripps Research Institute in California looked at mutations that could potentially take place in the H7N9 virus's genome.

They focused on the H7 hemagglutanin, a protein on the flu virus surface that allows it to latch onto host cells.

The team's findings, published in the journal PLoS Pathogens on Thursday, showed that in laboratory tests, mutations in three amino acids made the virus more able to bind to human cells - suggesting these changes are key to making the virus more dangerous to people.

Scientists not directly involved in this study said its findings were important, but should not cause immediate alarm.

"This study will help us to monitor the risk posed by bird flu in a more informed way, and increasing our knowledge of which changes in bird flu viruses could be potentially dangerous will be very useful in surveillance," said Fiona Culley, an expert in respiratory immunology at Imperial College London.

She noted that while "some of the individual mutations have been seen naturally, ... these combinations of mutations have not", and added: "The chances of all three occurring together is relatively low."

Wendy Barclay, a virologist and flu specialist also at Imperial, said the study's findings were important in showing why H7N9 bird flu should be kept under intense surveillance.

"These studies keep H7N9 virus high on the list of viruses we should be concerned about," she said. "The more people infected, the higher the chance that the lethal combination of mutations could occur."

(Editing by Tom Heneghan)