The United Nations agency has declared a global pandemic is underway from the virus known as swine flu which has so far been treatable with Tamiflu, made by Roche.
WHO spokesman Dick Thompson said that the discovery of a patient in Denmark whose infection did not respond to the drug, revealed by the Swiss company and Danish officials on Monday, did not amplify the severity of the virus.
"This is an isolated case. At this time, there is no public health implication. But we must remain alert as the virus can change at any time and we must not be complacent," he told Reuters.
Officials say the patient is now well and no further contagion with the resistant virus was detected.
Resistance to Tamiflu has been previously documented in the deadly bird flu virus H5N1 and seasonal H1N1 flu, Thompson said.
"We need to monitor the virus (H1N1) continuously," he said, adding that the WHO's global influenza surveillance network linking laboratories in 97 countries would keep monitoring it.
"WHO is not changing its recommendations for the use of antivirals," Thompson added, referring to the global body's advice to its 193 member states.
The WHO has previously said that the H1N1 virus is sensitive to a class of antiviral drugs which includes Tamiflu, known generically as oseltamivir, and Relenza by GlaxoSmithKline, known generically as zanamivir.
Denmark's State Serum Institute said, when making the announcement on Monday, that it was expected that the strain would at some point show resistance to Tamiflu.
It said that while the patient was found to be infected with a virus strain that had mutated to a form resistant to Tamiflu, the alternative drug Relenza, which is inhaled, remained an effective treatment.
"The person is well now and no further contagion with the resistent virus has been detected," it said in a statement.
