Researchers said more than 30 percent of H1N1 swine flu infection samples from northern Australia and more than 10 percent of those in Singapore, collected during the early months of 2011 had mildly reduced sensitivity to the two drugs.

There was no significant reduction in sensitivity to peramivir, an experimental flu drug from BioCryst Pharmaceuticals.

The new variant has also been detected in other parts of Asia-Pacific, according to a report by the World Health Organization (WHO).

H1N1 flu was discovered in Mexico and the United States in March 2009 and spread rapidly across the world. The WHO believes about 18,450 people died from the virus up to August 2010, including many pregnant women and young people. The WHO declared the pandemic over in August.

Swine flu has not gone away, however, and seasonal flu vaccines being offered across the world include for the H1N1 strain. Flu vaccines are made by several drugmakers including GlaxoSmithKline, Sanofi and Novartis.