The pandemic of swine flu may be hitting a peak in the Northern Hemisphere, global health officials said on Friday, but they cautioned it was far from over.

Officials also said they were investigating several troubling outbreaks of drug-resistant H1N1 but noted they were limited so far and that there were no indications yet the virus was mutating in a sustained way.

The World Health Organization said H1N1 flu was moving eastward across Europe and Asia after appearing to peak in parts of Western Europe and the United States.

At least 6,770 deaths have been recorded worldwide since the swine flu virus emerged in April, but officials always stress the confirmed count represents only a fraction of the actual cases, as most patients never get tested.

There are "early signs of a peak in disease activity in some areas of the northern hemisphere," the WHO said in a statement.

Transmission keeps intensifying in Canada, with the highest number of doctor visits by children. But U.S. officials saw signs of a slowdown.

WHO said Norway and countries farther east including Georgia, Lithuania, Moldova and Serbia were reporting sharp increases in influenza-like illness or acute respiratory infection.

Kazakhstan, Uzbekistan and parts of Afghanistan -particularly the capital, Kabul- are reporting higher numbers of flu cases. Israel is also reporting sharp increases.

"Essentially what is happening is that it is spreading eastward," Anthony Mounts of WHO's influenza team told Reuters. "Typically, seasonal influenza always starts west and moves eastward. It seems to be following that pattern except it is coming very early this year."

Drug resistant H1N1 found in U.S. and U.K.

British health officials said they were investigating the likely person-to-person spread of a drug-resistant strain of swine flu.

The Health Protection Agency reported five confirmed cases in Wales of patients infected with H1N1 resistant Roche AG and Gilead Sciences Inc's antiviral drug Tamiflu.

The patients had serious conditions that suppressed their immune systems, which can give the virus a better than usual opportunity to develop resistance, the HPA added.

U.S. CDC officials also said they were investigating four cases of H1N1 resistant to Tamiflu at Duke University hospital in North Carolina. "All four patients were very ill with underlying severely compromised immune systems and multiple other complex medical conditions," Duke said in a statement.

Health experts are looking for any sign that H1N1 is mutating into a drug-resistant form. Last year, the seasonal version of H1N1, a distant cousin of the pandemic strain, developed resistance to Tamiflu.

In Norway, officials were investigating a mutated strain in some patients that they said could be responsible for causing severe symptoms.

"The mutation could be affecting the virus' ability to go deeper into the respiratory system, thus causing more serious illness," the Norwegian Institute of Public Health said in a statement.

WHO said the mutation did not appear to be widespread in Norway and the virus remained sensitive to antivirals and pandemic vaccines.

If Tamiflu-resistant virus spreads widely, swine flu will become tougher to treat and may cost more lives, says Duke's Daniel Sexton, who is leading the hospital's investigation.

"The four patients involved in this situation had the same resistance pattern," says Sexton, adding that researchers at the Centers for Disease Control and Prevention are now testing virus samples from Duke to see whether they're identical.

"The resistance pattern suggests that might be the case," he says.

As of Friday, the World Health Organization has reported 57 cases of Tamiflu-resistant virus worldwide.

Agencies