H1N1 appears to have peaked in parts of western Europe and the United States, but transmission continues to intensify in Canada, the World Health Organization said Friday.

"In Canada, influenza transmission continues to intensify without a clear peak in activity," the UN health agency said.

Doctors visits for flu have been most common among children age five to 19 in Canada. The number of visits continues to exceed average rates for the past 12 flu seasons.

At least 6,770 deaths have been recorded worldwide since the H1N1 influenza A virus that causes swine flu emerged in April, according to the WHO. The agency reported 520 such deaths in the past week.

Spread of the flu appears to have peaked in western European countries, including Belgium, Britain, Iceland and Ireland after an intense period.

Norway and countries farther east such as Georgia, Lithuania, Moldova and Serbia are showing sharp increases in flu-like illnesses.

Kazakhstan, Uzbekistan and parts of Afghanistan, particularly Kabul, reported higher numbers of flu cases.

Flu transmission remains active in East Asia, including in Japan, where flu activity remains elevated but stable.

Mutation in Norway reported

WHO also said Friday it is investigating samples of mutated H1N1 virus linked to two deaths and one severe case of H1N1 flu in Norway, but so far, the significance is unclear.

"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," WHO said.

Norway's Institute of Public Health announced Friday that "the mutation could possibly make the virus more prone to infect deeper in the airways and thus cause more severe disease."

The mutation was found in three of 70 cases of swine flu, said Geir Stene-Larsen, the institute's director.

No additional instances have been detected, which suggests the mutation is not widespread in Norway, the WHO said. The same mutation has been found in fatal and mild cases in Brazil, China, Japan, Mexico, Ukraine, and the United States.

More clinical and epidemiological data is needed, said WHO spokesman Thomas Abraham.

Antiviral drugs continue to work against the virus with the mutation, and studies show current pandemic vaccines offer protection against it, both the WHO and the U.S. Centers for Disease Control and Prevention said.

Separately, health officials said four people in North Carolina have tested positive for Tamiflu-resistant swine flu.

It is the first cluster of this size in the U.S. The four cases were reported at Duke University Medical Center in Durham, N.C., over the past six weeks.

With files from The Associated Press