The World Health Organization says it is investigating possible mutations in the H1N1 swine flu virus following reports of mutations from at least six countries.
WHO pandemic influenza adviser Keiji Fukuda says the question is whether the reported mutations suggest a "fundamental change" in the virus. He says right now, health officials are not sure.
Fukuda commented Thursday in a briefing from Geneva. He says officials are trying to determine if the mutations are leading to changes in the virus' "clinical picture" -- which could result in more severe or less severe forms of swine flu.
The WHO adviser named Norway, Brazil, China, Japan, Ukraine and the United States as countries that have reported mutations. He also said world health officials are investigating some cases of drug-resistant swine flu.
Last week, WHO said more than 6,700 people have died from swine flu since the virus was first discovered in Mexico earlier this year.
Some information for this report was provided by AFP and AP.