The main flu virus circulating in the United States and Canada has shown "elevated resistance" to the antiviral drug Tamiflu, in line with findings in parts of Europe, the World Health Organisation (WHO) said on Friday.
The United Nations agency said it was too early to know what potential there may be for increased Tamiflu resistance in H5N1 avian influenza.
It did not change its recommendation that Tamiflu be used to treat human cases of bird flu.
A number of governments have been stockpiling Tamiflu, made by Switzerland's Roche Holding and Gilead Sciences of the United States, for use as a first line of defense in case birdflu sparks a human influenza pandemic.
The WHO said it was investigating the extent of resistance worldwide to Tamiflu, known generically as oseltamivir, in some seasonal H1N1 flu viruses that have a mutation making them "highly resistant."
The U.S. Centers for Disease Control and Prevention has reported a five percent prevalence of resistance to Tamiflu in samples of H1N1 virus tested to date. In Canada, 8 out of 81 samples showed resistance -- more than a 10 percent resistance rate, WHO spokeswoman Sari Setiogi said.
"These preliminary data indicate that oseltamivir resistance in H1N1 viruses is geographically variable but not limited to Europe," the WHO said in a statement.
A preliminary survey issued by the European Centre for Disease Control (ECDC) this week said that of 148 samples of influenza A virus isolated from 10 European countries during November and December, 19 showed signs of resistance to Tamiflu.
Of 16 samples from Norway, 12 tested positive for resistance against Tamiflu, according to the ECDC study.
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 said it was contacting national health authorities to determine the extent of resistance to the drug.
Neither Japan -- where Tamiflu is widely prescribed for seasonal flu -- nor Hong Kong had seen increased resistance to date, it said.
Past studies had found Tamiflu resistance rates ranging from zero to 0.5 percent, according to the U.N. agency.
"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.
Tamiflu has been proposed for treating H5N1 bird flu in humans.
Health experts fear that virus, which now mainly affects poultry, could mutate into a form that spreads easily among people and trigger a deadly pandemic.