Japan has confirmed its first case of a genetic mutation of the new H1N1 flu virus that shows resistance to Tamiflu, the main antiviral flu drug, a health ministry official said on Thursday.

But they said the mutated virus does not appear to be spreading and they see no immediate threat to public health.

They found the drug-resistant virus in a patient who was given Tamiflu to prevent infection but who developed symptoms anyway, the health ministry's Takeshi Enami said.

The World Health Organisation has declared a pandemic is under way from the virus, known as swine flu.

One of the biggest fears is that it will change into a form that resists the effects of the two main antiviral drugs, Tamiflu, made by Switzerland's Roche AG and GlaxoSmithkline's Relenza.

Seasonal flu is no longer affected by older flu drugs and certain strains resist Tamiflu's effects, leaving doctors with fewer options for protecting the most vulnerable patients.

Enami said it was not clear if Tamiflu would have helped the patient, whose age and sex were not given.

The patient was given Relenza and recovered, and does not appear to have infected anyone else.

Kyodo News Agency reported that the patient is a female teacher in her 40s.

Tamiflu, known generically as oseltamivir, and Relenza, known generically as zanamivir, can be used to prevent infection if given early enough. Both can treat flu but Tamiflu is often preferred because it is a tablet, while Relenza must be delivered using an inhaler.

"As of now, we have decided that this does not have a great impact in terms of public health," Enami said.

Only one other case of Tamiflu-resistant H1N1 flu has been found, in a patient in Denmark. WHO officials said the case was isolated.

Flu viruses are notoriously mutation-prone and flu experts say it would not be at all surprising if this new H1N1 flu developed resistance.

"It was not something not to be expected," Roche spokeswoman Martina Rupp said. "This is line with what we already know from clinical trials."

Japan has confirmed 1,351 cases of the H1N1 flu to Wednesday, the health ministry website showed. The World Health Organisation has confirmed more than 77,000 cases of the new H1N1 flu and 332 deaths. But U.S. officials say there are likely at least a million cases there alone.