The main flu virus circulating in Canada and the United States has shown "elevated resistance" to the antiviral drug Tamiflu, the World Health Organization said on Friday.

In Canada, 8 out of 81 samples showed resistance -- more than a 10 per cent resistance rate, a WHO spokesperson said

Normally, just under one per cent of flu viruses would be expected to be resistant to the drug.

"These preliminary data indicate that oseltamivir resistance in H1N1 viruses is geographically variable but not limited to Europe," the WHO said in a statement.

The U.S. Centers for Disease Control and Prevention have reported a five per cent rate of resistance to Tamiflu in samples of H1N1 virus tested to date.

At the same time, the European Centre for Disease Control is reporting that flu viruses resistant to Tamiflu, known generically as oseltamivir, have been found in nine of 17 European countries.

About 14 per cent of H1N1 flu viruses checked carried the resistance mutation, which has been shown in testing to dramatically reduce a virus's susceptibility to the drug, the agency said.

The new "elevated resistance to oseltamivir" appears limited to seasonal H1N1 viruses, and does not involve H3N2 or influenza B viruses which are also circulating, the WHO said.

"This means that oseltamivir would most likely be ineffective for treating or preventing infections caused by these resistant H1N1 strains, although the drug will be effective against other influenza virus infections," it added.

The WHO adds that there is no evidence that the resistant viruses are causing more severe illness than other influenza viruses.

The agency said it was contacting national health authorities to determine the extent of resistance to the drug.

Surprisingly, neither Japan -- where Tamiflu is widely prescribed for flu -- nor Hong Kong had seen increased resistance to date, it said.

Many of the viruses tested in Europe were from the early part of the flu season. It's possible the pattern might have shifted since then, or the situation may worsen as the flu season continues.

The WHO said it cannot explain this Tamiflu resistance.

"The frequency of oseltamivir resistance in H1N1 viruses in the current influenza season is unexpected and the reason why a higher percentage of these viruses are resistant is currently unknown," the WHO said.

"Influenza viruses are continuously changing and it is possible that a resistant strain has emerged spontaneously. Further detailed laboratory characterization of circulating H1N1 viruses and epidemiological information on patients will be needed to help answer this question."

The resistance mutation has been found virtually exclusively in viruses collected from people who had not taken Tamiflu. That suggests the drug may be more vulnerable to the development of resistance than initially believed.

Tamiflu is seen as the main medication for treating H5N1 bird flu in humans.

It has also been proposed as the main defence in the event that that virus, which now mainly affects poultry, mutates into a form that spreads easily among people.

The WHO's Global Influenza Program, held a teleconference of about 50 scientists from leading influenza laboratories around the world earlier this week, to try to get a handle on how far this resistance has spread and what is driving the spread.