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