Cases of the H1N1 pandemic influenza virus showing resistance to antiviral drugs are isolated and infrequent, the World Health Organization said on Thursday.

We see no evidence at all that there is widespread occurrence of antiviral resistance, the U.N. health agency's pandemic flu expert, Keiji Fukuda, told a news conference.

Antiviral drugs such as oseltamivir, marketed by Swiss drugmaker Roche Holding as Tamiflu, have encountered only sporadic resistance fromH1N1, known as swine flu, he said.