A strain of H5N1 which can apparently sidestep defences of existing vaccines, is spreading in China and Vietnam, the UN Food and Agriculture Organisation said in a statement. It urged greater surveillance to ensure any outbreaks are contained.
Last week, the World Health Organisation reported that a six-year-old Cambodian girl had died on August 14 from bird flu, the eighth person to die from H5N1 avian influenza this year in Cambodia.
Vietnam suspended its spring-time poultry vaccination this year, the FAO said. Most of the northern and central parts of the country where the virus is endemic have been invaded by the new strain.
Elsewhere, the FAO says bird migrations over the past two years have brought H5N1 to countries that had been virus-free for several years, including Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia.
"Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it," said the FAO's chief veterinary officer Juan Lubroth.
WHO says globally there have been 331 human deaths from 565 confirmed bird flu cases since 2003 when it was first detected.
The virus was eliminated from most of the 63 countries infected at its peak in 2006, but it remained endemic in six countries: Bangladesh, China, Egypt, India, Indonesia and Vietnam.
The number of outbreaks in poultry and wild bird populations shrank from a high of 4,000 to 302 in mid-2008, but outbreaks have risen progressively since, with almost 800 cases reported in 2010-11, the FAO said.
"The general departure from the progressive decline in 2004-08 could mean that there will be a flare-up of H5N1 this autumn and winter, with people unexpectedly finding the virus in their back yard," Mr Lubroth.