Avian flu shows signs of a resurgence, while a mutant strain - able to sidestep vaccines - could be spreading in Asia, the United Nations has warned.

The variant appeared in Vietnam and China and its risk to humans cannot be predicted, veterinary officials said.

Virus circulation in Vietnam threatens Thailand, Malaysia and Cambodia, where eight people have died after becoming infected this year, they warned.

The World Health Organization says bird flu has killed 331 people since 2003.

It has also killed or provoked the culling of more than 400m domestic poultry worldwide and caused an estimated $20bn (£12.2bn) of economic damage.

Wild birds

The virus had been eliminated from most of the 63 countries infected at its 2006 peak, which saw 4,000 outbreaks across the globe, but remains endemic in Bangladesh, China, Egypt, India, Indonesia and Vietnam.

And the number of cases has been rising again since 2008, apparently because of migratory bird movements, said the UN's Food and Agriculture Organization (FAO) chief veterinary officer, Juan Lubroth.

"Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it," he said.

Avian flu has in the past two years appeared in poultry or wild birds in countries that had been virus-free for several years: Israel and the Palestinian Territories, Bulgaria, Romania, Nepal and Mongolia are among those recently affected.

Mr Lubroth said the new strain had infected most parts of northern and central Vietnam and could also pose a risk to Japan and the Korean peninsula.

South Korea began culling hundreds of thousands of chickens and ducks in December last year after confirming its first cases since 2008.

The FAO is calling for countries to adopt "heightened readiness and surveillance" against a resurgence of the virus.

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