The UN health agency said it would not change its disease alert level after the virus appeared in wildfowl and farm birds in Turkey. The top level, six, represents a lethal pandemic.

"The WHO level of pandemic alert remains unchanged at phase 3: a virus new to humans is causing infections, but does not spread easily from one person to another," it said in a statement.

"The spread of H5N1 to poultry in new areas is of concern as it increases opportunities for further human cases to occur."

"However, all evidence to date indicates that the H5N1 virus does not spread easily from birds to infect humans," the statement added.

Tests have confirmed the presence of the highly pathogenic strain in samples taken from birds in Turkey, triggering alerts from authorities and a rush on anti-flu drugs or vaccines.

The H5N1 strain, which has caused at least 117 cases in humans and more than 60 deaths in Asia since 2003, according to the WHO, appears to be spreading westwards with migrating birds.

Scientists fear it could mutate into a form that passes easily from human to human, triggering a global flu outbreak that could lead to millions of deaths.

Romanian authorities are awaiting the results of tests on birds thought to be infected with avian flu.

While it advised people to avoid contact with poultry in infected areas and reiterated other preventive measures, the WHO reacted cautiously to reports that pharmacies were being stripped clean of anti-flu drugs, notably in France and in Romania.

"It reflects a misunderstanding, but it's an entirely reasonable misunderstanding because we've been saying bird flu pandemic," WHO spokesman Dick Thompson told AFP.

"They're almost different diseases. Maybe we should have been a little bit clearer about saying this is a virus of pandemic potential and very rarely does this virus jump into humans," he added.

The WHO believes a genetic mix of the H5N1 strain of bird flu with other forms of the virus in animals like pigs, or in humans, could produce another strain of the disease that is far more infectious and lethal than seasonal types of influenza.

"What would emerge would be a disease as deadly as avian influenza, which currently has a case fatality rate of over 50 percent, but that moves as easily as seasonal influenza does from person to person," Thompson said.

Scientist believe a similar process triggered global flu pandemics in the past century that caused millions of deaths.

"The widespread concern may be bad communication on our part. Millions of animals have been infected and very rarely have humans been infected," Thompson underlined.

Asia's dense human population and close proximity with farm animals like ducks were key reasons for the persistent spread of the bird flu virus there in recent years, WHO Western Pacific director Shigeru Omi said.

Governments in Asia and Europe, as well as the WHO, are assembling stockpiles of anti-flu drugs, which are regarded as one of the lines of defence against a human pandemic.

Thompson cautioned that the drugs, including Roche's Tamiflu and GlaxoSmithKline's Relenza, as well as current flu vaccines, had limited scope.

"Oseltamivir has been effective in killing the (H5N1) virus in laboratory tests, however in people it has had rather mixed results," Thompson said, underlining that drugs needed to be administered early on in the illness.

Oseltamivir is the main ingredient of Tamiflu.

"The current seasonal influenza vaccine as being offered, as with all seasonal vaccines, would not be effective, it would not provide protection from avian influenza," he added.

The vaccine is nonetheless recommended for vulnerable people and those dealing with bird flu outbreaks, partly because it would reduce the chance of bird flu mixing with the more benign human influenza viruses.