The United Nations warned on 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 defenses of existing vaccines, is spreading in China and Vietnam, the UN Food and Agriculture Organization (FAO) said in a statement on Monday. It urged greater surveillance to ensure that any outbreaks are contained.

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

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, Bulgaria, Romania, Nepal and Mongolia.