Japan and Hong Kong say they have each identified cases of the swine influenza A-H1N1 virus that are resistant to the anti-viral drug Tamiflu. Denmark reported the first such case earlier this week.
Japan's Health Ministry said Friday that it discovered the resistance in a patient who had been given Tamiflu after being diagnosed with swine flu. The patient was recovering after taking Relenza, another medicine used to contain the pandemic.
Hong Kong health officials say they found the Tamiflu-resistant strain in a sample from a 16-year-old girl who tested positive for swine flu when she returned from the city of San Francisco in the U.S. state of California last month.
The company that makes Tamiflu, Swiss drug maker Roche says about half of one percent of all flu cases are resistant to the drug.
The World Health Organization says more than 77,000 cases of swine flu have been confirmed worldwide, including more than 330 deaths.
The WHO recently declared a pandemic - the first in more than 40 years - as the H1N1 swine flu spread worldwide.
Some information for this report was provided by AFP, AP and Reuters.