Staff wearing protective clothing cull chickens in a Sham Shui Po market after the deadly H5N1 bird flu virus was found in samples collected from the market's poultry stalls in Hong Kong, on June 7, 2008. The United Nations warned on August 29, 2011 that avian flu shows signs of a resurgence, and a mutant strain could be spreading in China and Vietnam. (ANDREW ROSS/AFP/Getty Images)

Bird flu is showing signs of returning, and a mutant strain of the deadly H5N1 virus could be spreading in Asia, the United Nations has warned. 

The UN Food and Agriculture Organization issued a warning Monday that a mutant strain of bird flu could be spreading to previous virus-free countries by wild bird migrations, CBS News reports.

The FAO says the new strain can apparently dodge existing avian influenza vaccines, and is spreading in China and Vietnam. The organization urged greater surveillance of outbreaks, CBS News says.

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Most of north and central Vietnam, where bird flu is endemic, have been invaded by the mutant strain, according to the FAO. The risk to humans cannot yet be predicted, health officials told the BBC.

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

Bird flu has also remained endemic in Bangladesh, China, Egypt, India and Indonesia, but was eliminated from most of the 63 countries infected at its peak in 2006, CBS News says.

The FAO says wild bird migrations over the past two years have brought the H5N1 avian influenza to countries that were previously virus-free, including Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia.

Bird flu is found in both wild birds and domestic birds such as chickens and ducks, and can spread to people who have close contact with infected birds, although not by eating poultry or eggs that have been properly cooked.

"Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it," the UN agency's chief veterinary officer, Juan Lubroth, said in a statement.

"The general departure from the progressive decline in 2004-2008 could mean that there will be a flare-up of H5N1 this fall and winter, with people unexpectedly finding the virus in their backyard," Lubroth said.

The World Health Organization reported last week that a 6-year-old Cambodian girl had died from bird flu, the eighth person to die from the H5N1 virus this year in Cambodia.

The WHO says bird flu has killed 331 people globally since 2003 when it was first detected.