Bird flu is showing signs of returning due to wild bird migrations, and a mutant strain of the deadly H5N1 virus has started spreading in Asia.

The United Nations' Food and Agriculture Organization (FAO) has issued a warning, looking at the situation that it a mutant strain of deadly flu is already spreading in China and Vietnam, according to CBS News report.

Fears of an outbreak of bird flu this winter have been raised by the United Nations. Bird flu was in decline, but FAO warns it appears to be on the rise again and the new bird flu strain is even deadlier.

It said that bird migrations over the past two years have brought H5N1 to countries that for years had been virus-free, including Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia.

Recently, WHO reported that a 6-year-old Cambodian girl died Aug. 14 from bird flu, and about eight registered cases of H5N1 avian influenza are in fatal state.

Existing vaccines appear to be powerless against the new strain of the H5N1 virus as it has now spread across China and most of the northern and southern parts of Vietnam, even though Vietnam suspended its springtime poultry vaccination this year, FAO said.

The WHO report states that globally, there have been 331 human deaths from 565 confirmed bird flu cases since 2003.

Wild birds may introduce the virus, but people's actions in poultry production and marketing spread it, said Juan Lubroth, the FAO's chief veterinary officer.

The H5NI virus is known to be endemic in six countries including Bangladesh, China, Egypt, India, Indonesia and Vietnam, but no country can consider itself safe, he said.

The symptoms of bird flu in humans are regular to those of regular influenza, so spotting it at early stage gets difficult. The symptoms include fever, sore throat, cough, headache, and muscle ache.