UPDATE 2-WHO reports Tamiflu-resistant flu in U.S., Canada

(Adds fresh figures on Canada in para 8, quote in 15th para)

By Stephanie Nebehay

GENEVA, Feb 1 (Reuters) - The main seasonal flu virus in the United States and Canada as well as parts of Europe shows higher resistance to the antiviral drug Tamiflu, raising questions about its potential effectiveness in a human bird flu pandemic.

The World Health Organisation (WHO) reported the elevated resistance in North America on Friday, but said it was too early to know what the chances may be for increased Tamiflu resistance in the H5N1 strain of 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 AG (ROG.VX: Quote, Profile, Research) and Gilead Sciences Inc (GILD.O: Quote, Profile, Research) of the United States, for use as a first line of defence in case bird flu sparks a human influenza outbreak.

Health experts fear that the virus, which now mainly affects poultry, could mutate into a form that spreads easily among people and trigger a deadly 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 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.