The World Health Organisation said Thursday that it is investigating reports of mutations in the swine flu virus, after half-a-dozen countries recorded cases in which the virus was transforming.

"The question is whether these mutations again suggest that there is a fundamental change going on in viruses out there - whether there's a turn for the worse in terms of severity," said Keiji Fukuda, WHO's special adviser on pandemic influenza.

"The answer right now is that we are not sure," he added following reports from China, Japan, Norway, Ukraine and the United States.

He noted, however, that mutations are common in influenza viruses, and "if every mutation is reported out there it would be like reporting changes in the weather."

"What we're tryin to do when we see reports of mutations is to identify if these mutations are leading to any kinds of changes in the clinical picture - do they cause more severe or less severe disease?

"Also we're trying to see if these viruses are increasing out there as that would suggest a change in epidemiology," he added.

At the moment, the mutated A(H1N1) virus has been detected both in people with more severe and milder diseases. The "question is whether it's associated with severe diseases more often," said Fukuda.

China said earlier Thursday that it had discovered eight people with mutated versions of swine flu while Norway reported last week that it had detected one case.

Fukuda also said that the UN health agency was looking into Tamiflu-resistant cases reported in Britain and the United States but noted they concerned people who are already undergoing treatment for other diseases or who have underlying health issues.

The health agency was therefore maintaining its assessment that Tamiflu, produced by Swiss drugmaker Roche, remained "effective" as a treatment for swine flu, but that "we do have to be vigilant in these very susceptible people."