ZURICH/COPENHAGEN - Scientists have established the first case of the new H1N1 influenza strain showing resistance to Tamiflu, the main antiviral flu drug, Danish officials and the manufacturer said on Monday.

"While receiving the drug, the patient appeared to develop resistance to it," David Reddy, Roche Holding AG's pandemic taskforce leader, told reporters on a conference call on a case observed in Denmark.

"This is the first report we have of it in H1N1."

Common seasonal flu can resist Tamiflu and Reddy said a case of resistance in H1N1 - also know as swine flu - was not unexpected. Roche had been working on strategies to counter such a development.

The World Health Organization declared an influenza pandemic earlier this month and advised governments to prepare for a long-term battle against an unstoppable new flu virus. The WHO had no immediate comment on the case of Tamiflu resistance.

The United Nations agency raised its pandemic flu alert to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way.

"The person is well now and no further contagion with the resistent virus has been detected," Denmark's State Serum Institute said in a statement.

Roche shares closed up 1.4 percent at 149 Swiss francs ($199), outperforming a 0.7 percent firmer DJ Stoxx European drugs index .

This case does not change the recommendation to use Tamiflu, the institute said.

The U.S. Centers for Disease Control and Prevention has expressed concern the new strain may mix with seasonal flu strains, which are still circulating.

The WHO has said Tamiflu was working against strains of the new H1N1 flu but some analysts have expressed concern it might be less effective than Relenza, GlaxoSmithKline's inhaled drug, since there have been widespread reports of resistance by seasonal H1N1 flu.

The seasonal strain of H1N1 is a distant cousin of the swine flu and was widely resistant to Tamiflu this year.