A mutation had been found in samples of the swine flu virus taken following the first two deaths from the pandemic in Norway, the WHO said on Friday.
However, it stressed that the mutation did not appear to cause a more contagious or more dangerous form of A(H1N1) influenza and that some similar cases observed elsewhere had been mild.
"The Norwegian Institute of Public Health has informed WHO of a mutation detected in three H1N1 viruses," the WHO said in a briefing note.
"The viruses were isolated from the first two fatal cases of pandemic influenza in the country and one patient with severe illness," it said, although it added that no further instances were found in tests.
"Norwegian scientists have analysed samples from more than 70 patients with clinical illness and no further instances of this mutation have been detected. This finding suggests that the mutation is not widespread in the country," the UN health agency explained.
WHO spokesman Gregory Haertl told AFP that the global health watchdog did not believe "that this has any significant impact for the time being."
However, the agency revealed that a similar mutation had been observed in Brazil, China, Japan, Mexico, Ukraine, and the United States, as early as April.
"The mutations appear to occur sporadically and spontaneously. To date, no links between the small number of patients infected with the mutated virus have been found and the mutation does not appear to spread," the statement said
Some of those cases also produced mild symptoms, Haertl noted.
The WHO also underlined that there was no evidence of more infections or more deaths as a result, while the antiviral drugs used to treat severe flu, oseltamivir (Tamiflu) and zanamivir (Relenza), were still effective on the mutated virus.
"Studies show that currently available pandemic vaccines confer protection," it added, as mass vaccine campaigns were slowly gaining ground in the northern hemisphere amid signs of public skepticism in several European nations.
Scientists fear that mutations in flu viruses could cause more virulent and deadly pandemic flu. The global health watchdog reiterated a call for close monitoring.
The WHO was still assessing the significance of the latest observation, but it stressed that many such changes in the flu virus do not alter the illness it causes in patients.
"Although further investigation is under way, no evidence currently suggests that these mutations are leading to an unusual increase in the number of H1N1 infections or a greater number of severe or fatal cases," it added.
Norwegian authorities reported the country's first swine flu death on September 3, a 52 year-old Danish truck driver who died just over a week earlier.
On Friday, WHO data showed reported that around 6,750 people had died from swine flu since the virus was first uncovered in Mexico and the United States in April.
That represented an increase of about 500 more than a week ago, as the pandemic took hold in the northern hemisphere during the cold season
The WHO estimates that some 250,000 to 500,000 people die every year from standard seasonal variants of swine flu.