GENEVA, Nov. 24 (Xinhua) -- The World Health Organization said Tuesday that it was looking at a mutation of the A/H1N1 flu virus recently detected in several countries and regions "very carefully" to see whether it causes severe diseases.

    So far, there was still no evidence suggesting the mutation, most recently found in Hong Kong of China and Norway, is associated with severe cases of infection, the WHO said.

    "We really need to look at this very carefully to see whether it is in fact associated with severe cases," said WTO spokesman Thomas Abraham.

    Abraham said investigations would be done through the WHO's collaborating network of laboratories and "through understanding more about clinical features associated with the infection of this particular form of the virus."

    According Abraham, there was currently no evidence suggesting the mutated form of the A/H1N1 flu virus was spreading. The mutations appeared to occur sporadically and spontaneously.

    Norwegian health authorities last week informed the WHO of a mutation of the A/H1N1 flu virus detected in two patients who died and one with severe illness.

    Hong Kong's Department of Health announced Monday that it also had found the same mutation in an A/H1N1 flu virus sample.

    The WHO said similar mutations had been found since April and they involved both severe and non-severe cases. Countries that have reported the mutation include the United States, Mexico, Ukraine, Brazil, Japan and China.

    Like all other flu viruses, the A/H1N1 virus had been changing constantly, Abraham said.

    He said the vast majority of mutations have no clinical significance at all, but "occasionally we come across a virus that might have clinical significance."

    The A/H1N1 virus has been stable in the sense of clinical features and the illness that it produces, Abraham added.

    It has been "remarkably stable" also in the fact that both antiviral drugs, oseltamivir and zanamivir, and the currently available pandemic vaccines are effective, he said.