A patient with H1N1 influenza in Denmark showed resistance to Roche Holding AG´s Tamiflu, the main antiviral flu drug, a company executive said on Monday.

"While receiving the drug, the patient appeared to develop resistance to it," David Reddy, Roche´s pandemic taskforce leader, told reporters on a conference call on the Danish case. "This is the first report we have of it in H1N1."

The World Health Organization has raised its pandemic flu alert on the H1N1 flu virus to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way.

Common seasonal flu can resist Tamiflu and Reddy said a case of resistance in H1N1 was not unexpected, adding Roche has been working on strategies to counter such a development.