The United Nations is warning of a possible resurgence of the deadly bird flu virus, including a mutant strain that is spreading in Asia and elsewhere.

The world body's Food and Agriculture Organization said Monday that a variant strain of the H5N1 virus , which appears able to bypass the defenses of existing vaccines, has appeared in China and Vietnam.

The agency also said bird flu has killed eight people in Cambodia this year, including a 6-year-old girl who died this month.

The H5N1 virus was first detected in 2003, and by the peak of the outbreak in 2006, had spread to 63 nations, before it was eradicated in most of those countries due to a mass culling of domestic poultry.

But the virus continued to infect people in Bangladesh, China, Egypt, India, Indonesia and Vietnam. The agency says there were almost 800 cases of bird flu reported between 2010 and 2011.

The FAO says the virus has spread since 2008 due to the migration patterns of wild birds across a wide geographic area, including Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia, which had been virus-free for several years.

The FAO's chief veterinary officer, Juan Lubroth, says that while wild birds may introduce the virus, “people's actions in poultry production and marketing spread it.” He says the resurgence of H5N1 could mean people “unexpectedly finding the virus in their backyard” in the coming months.

The U.N.'s World Health Organization says 565 people have been infected since the virus was first detected, with 331 dying as a result.