GENEVA (Reuters) - The main seasonal flu virus in theUnited States and Canada as well as parts of Europe showshigher resistance to the antiviral drug Tamiflu, raisingquestions about its potential effectiveness in a human bird flupandemic.

The World Health Organisation (WHO) reported the elevatedresistance in North America on Friday, but said it was tooearly to know what the chances may be for increased Tamifluresistance in the H5N1 strain of avian influenza.

It did not change its recommendation that Tamiflu be usedto treat human cases of bird flu.

A number of governments have been stockpiling Tamiflu, madeby Switzerland's Roche Holding AG and Gilead Sciences Inc ofthe United States, for use as a first line of defense in casebird flu sparks a human influenza outbreak.

Health experts fear that the virus, which now mainlyaffects poultry, could mutate into a form that spreads easilyamong people and trigger a deadly pandemic.

The WHO said it was investigating the extent of resistanceworldwide to Tamiflu, known generically as oseltamivir, in someseasonal H1N1 flu viruses that have a mutation making them"highly resistant."

"The frequency of oseltamivir resistance in H1N1 viruses inthe current influenza season is unexpected and the reason why ahigher percentage of these viruses are resistant is currentlyunknown," the WHO said.

The U.S. Centers for Disease Control and Prevention hasreported a five percent prevalence of resistance to Tamiflu insamples of H1N1 virus tested. In Canada, 8 out of 128 samplesshowed resistance, roughly 6 percent, WHO spokeswoman GregoryHartl said.

"These preliminary data indicate that oseltamivirresistance in H1N1 viruses is geographically variable but notlimited to Europe," the WHO said in a statement.

A preliminary survey issued by the European Centre forDisease Control (ECDC) this week said that of 148 samples ofinfluenza A virus isolated from 10 European countries duringNovember and December, 19 showed signs of resistance toTamiflu.

The mutated H1N1 is a sub-type of influenza A.

Of 16 samples from Norway, 12 tested positive forresistance against Tamiflu, according to the ECDC study.

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

"This means that oseltamivir would most likely beineffective for treating or preventing infections caused bythese resistant H1N1 strains, although the drug will beeffective against other influenza virus infections," it added.

The WHO said it was contacting national health authoritiesto determine the extent of resistance to the drug. NeitherJapan -- where Tamiflu is widely prescribed for seasonal flu --nor Hong Kong had seen increased resistance to date, it said.

"It is still early in the (seasonal flu) season, we don'thave a full picture yet," Hartl said.

Past studies had found Tamiflu resistance rates rangingfrom zero to 0.5 percent, according to the U.N. agency.

(Reporting by Stephanie Nebehay; Editing by Caroline Drees)