Virologists warned on Tuesday that there was no vaccine against a mutant strain of H5N1 bird flu now spreading in China and Vietnam and called for closer monitoring of the disease in poultry and wild birds to stop it spreading to people.

The call came after the U.N. Food and Agriculture Organization (FAO) warned on Monday of a possible resurgence of bird flu and said a mutant strain of the H5N1 was spreading in Asia and beyond.

While scientists are uncertain if this new strain -- called H5N1- -- is more virulent in people, they said it was different enough from its predecessor to escape a human H5N1 vaccine that can tackle the parent strain.

"There is a human H5N1 vaccine candidate that is a (WHO)recommended vaccine ... But it doesn't confer full protection against the (new variant)," said leading virologist Malik Peiris at the University of Hong Kong.

"But that is not unusual. H5 viruses keep changing and we have to change the vaccine strain."

The World Health Organization meets twice a year, in February and September, when experts discuss and decide on the makeup of candidate influenza vaccines.

H5N1 kills up to 60 percent of the people it infects. It has resurfaced in recent months, most notably in Cambodia where it has infected eight people this year, killing all of them.

"H5N1 cases in Cambodia always have high mortality because they are detected late," Peiris told Reuters in an interview.

"It doesn't necessarily indicate that this particular virus strain is more virulent to humans. But it is a threat because it has become more widespread globally."