The World Health Organisation (WHO) on Tuesday voiced concern about the spread of bird flu in Vietnam, but said it was not considering warning against travel there as the risk of human infection was low.

Five people have died in Vietnam since late December, taking deaths there to 25 since 2003, more than twice the toll in neighbouring Thailand, the second most affected Asian country.

Vietnamese authorities have told the United Nations agency that up to 10 additional suspected human cases are under investigation and that the disease has spread to all regions of the country, WHO spokeswoman Maria Cheng told Reuters.

“Our concern is that it looks like avian flu has spread,” she said. “There are a number of (human) cases under investigation across the country.”

But at this stage, there was no need for any warning. “The WHO is not considering any travel advisory at the moment. It is a pretty low risk to public health because it requires close contact with chickens,” she said.

The WHO slapped travel advisories against parts of China and other countries after a new flu-like disease SARS (severe acute respiratory syndrome) emerged in the first half of 2003.

The H5N1 virus made the first known jump from bird to humans in Hong Kong in 1997, killing six people, and any outbreak of the disease among poultry raises fears of human cases.

The WHO has warned that the virus could mutate to an even more lethal form for humans and cause a global human pandemic.