Scientists have discovered two spots on the H5N1 bird flu virus which would need to mutate for the virus to infect people more easily.

H5N1 is largely a disease in animals because it has surface proteins which bind more easily to "receptors" lining respiratory tracts of birds, rather than receptors in humans.

Experts fear H5N1 will infect more humans and trigger a pandemic killing millions of people if the virus mutates to attach easily to human receptors.

In the latest issue of Nature, scientists in Japan, Britain and the United States said they had discovered two specific spots on the genes of H5N1 which appeared to determine if the virus attached more easily to bird or human receptors.

This discovery will help scientists decide if any strain of the H5N1 has the potential to cause a human pandemic.

"The bottom line is that the changes (on the two spots) can be used as molecular markers to identify the potential of the viruses that may grow well in humans," said Yoshihiro Kawaoka of the Institute of Medical Science at the University of Tokyo.

Using 21 samples of the H5N1 virus taken from human victims in Indonesia and Vietnam, the team of scientists found that three of them bound especially easily to human receptors.

"We found many mutations and we tried to identify which mutations were important ... two appeared to be very important," Kawaoka told Reuters by telephone from the United States.

Kawaoka warned against any over-emphasis on these two spots.

"It is very important that we shouldn´t only focus on these two. The virus can become human-like by many mutations, these two are important but they are not the only ones," he said.

"But these two will give indication when a virus has changed receptor specificity."