Fears of a fresh outbreak of bird flu this winter have been raised by the United Nations, after an increase in the number of deaths and, crucially, the emergence of a new, mutated strain of the disease.

At least eight people have died of bird flu in Cambodia this year, the most recent being a six-year-old girl earlier this month, and the virus has reached countries that had been free of it for several years.

Existing vaccines appear to be powerless against the new strain of the H5N1 virus which, the UN's Food and Agriculture Organisation (FAO) said, has now spread across much of Vietnam and China. It remains uncertain whether the mutant virus can be transmitted to humans, and if so how dangerous it potentially is.

The FAO has urged stiffer surveillance measures to prevent the disease spreading to new areas. "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.

And he warned: "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 backyard."

Some countries which were previously free of the virus have suffered outbreaks over the past two years as a result of it being introduced by migratory birds. These include Bulgaria, Romania, Israel, the Palestinian territories, Nepal and Mongolia.Bird flu was first detected in 2003 and 331 deaths worldwide have been attributed to it from 565 cases of bird flu in humans. At its peak in 2006 it was present in 63 countries and there were 4,000 outbreaks of the disease in wild birds and poultry.

Measures to halt the spread of the disease included widespread culls of poultry and other birds where infections were found or suspected.

In all, some 400 million domestic poultry were slaughtered and the disease was said to have cost the world's economies $20bn but the spread was halted. In the UK exclusion zones were set up around domestic and wild bird populations where the virus was detected and many thousands of birds were culled.

The most high-profile case in Britain was that of a Bernard Matthews turkey farm in Holton, Suffolk, where 160,000 turkey chicks were gassed to prevent the virus spreading any further. The precise route by which the virus reached the farm was never categorically established.

By 2008 the number of cases had been brought down dramatically to 302, but since then the numbers have been creeping up again, with almost 800 cases reported around the world in the past year.

In six countries – Bangladesh, China, Egypt, India, Indonesia and Vietnam – it is known to be endemic.