The UN warns that wild bird migrations have brought a mutant strain of bird flu back to some countries that were previously virus-free.The UN warns that wild bird migrations have brought a mutant strain of bird flu back to some countries that were previously virus-free. Kin Cheung/ Associated Press

The United Nations warned Monday of a possible resurgence of the deadly bird flu virus, saying wild bird migrations had brought it back to previously virus-free countries and that a mutant strain was spreading in Asia.

A mutant strain of H5N1, which can apparently sidestep defences of existing vaccines, is spreading in China and Vietnam, The UN Food and Agriculture Organization said in a statement Monday. It urged greater surveillance to ensure that any outbreaks are contained.

Last week, the World Health Organization reported that a 6-year-old Cambodian girl had died Aug. 14 from bird flu, the eighth person to die from H5N1 avian influenza this year in Cambodia.

Vietnam suspended its springtime poultry vaccination this year, 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, 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.