The new strain has appeared in Vietnam and China, and represents a possible threat to humans, the World Health Organization said.
Eight people have died after becoming infected so far this year, the BBC reported Monday.
While the virus had been eliminated from many countries infected in 2006 at the outbreak's peak, it remains endemic in Bangladesh, China, Egypt, India, Indonesia and Vietnam.
The new strain has infected most parts of northern and central Vietnam and threatens Thailand, Malaysia and Cambodia, the U.N.'s Food and Agriculture Organization chief veterinary officer Juan Lubroth said.
The number of cases has been rising again since 2008, apparently because of migratory bird movements, he said.
"Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it," he said.
The FAO has urged countries to adopt "heightened readiness and surveillance" against the virus.
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