A new and deadly strain of the bird flu virus may be on the rise again.

The United Nations is warning of the possible resurgence of a strain known as H5N1, saying wild bird migrations brought the flu back to previously virus-free countries and that a mutant strain was spreading in Asia.

The World Health Organization has reported a 6-year-old Cambodian girl died from bird flu on Aug. 14. The girl was the eighth person to die from this particular strain of the virus.

Health officials say this virus can sidestep the defenses of existing vaccines.

The U.N. Food and Agriculture Organization said it is spreading in China and Vietnam. They suggested that problems with poultry farms may be to blame for its spread.

Vietnam suspended its springtime 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.

FAO said 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 FAO Chief Veterinary Officer Juan Lubroth. He is urging greater preparedness and surveillance.

"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 in a statement.