The World Health Organization is looking into reports in Britain and the United States that the H1N1 flu may have developed resistance to Tamiflu in people with severely suppressed immune systems, a spokesman said Tuesday.

Britain's Health Protection Agency (HPA) said five cases have been confirmed in Wales of patients infected with H1N1resistant to oseltamivir -- the generic name of Roche 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 said. It said the drug-resistant strain had probably spread person to person.

We have seen the reports, we need to look into them, WHO spokesman Thomas Abraham said in Geneva.

The U.S. Centers for Disease Control and Prevention last week also reported four cases of H1N1 resistant to Tamiflu at Duke University Hospital in North Carolina. All were said to be very ill with underlying severely compromised immune systems and multiple other complex medical conditions.

The WHO spokesman said both the reports involved Tamiflu resistance in people with severely compromised immune systems.

We'll see if we need to put any additional measures in place to protect this vulnerable group of patients. It might mean that they are at more serious risk than others, Abraham said.

People with suppressed immune systems, such as those undergoing chemotherapy or suffering from HIV are more likely to fall ill from infections.

The WHO has previously reported cases of the pandemic virus being resistant to oseltamivir but says these are rare.

Abraham, asked whether the cases in Wales would be the first instance of person-to-person transmission of a Tamiflu-resistant form, replied: As far as I know there have been possibilities but it never has been conclusively shown.

H1N1, a mixture of swine, bird and human viruses, has killed at least 6,770 people globally, according to the WHO. Most people suffer mild symptoms such as aches or fever, but recover without special treatment, it says.

Separately, the WHO said it was still probing whether a mutation in theH1N1 influenza strain, detected most recently in Norway last week, is causing the severest symptoms among those infected.

The Norwegian Institute of Public Health said last Friday the mutation could affect the virus' ability to go deeper into the respiratory system, causing more serious illness.

It is a major issue we are looking at, Abraham said.

If the mutation in fact is associated with severe cases then we really need to know about it. This might be a signal. We need to investigate, he said. As of now there is no evidence of a particular association with severe cases.

So far, antiviral drugs and vaccines have been effective against the mutated form, he said.

There have now been four cases of mutated virus in patients in Norway, following a similar mutation in H1N1 viruses circulating in several other countries since April, he said.

The other countries are Brazil, China, Japan, Mexico, Ukraine and the United States.

What we've seen has been pretty much equal in terms of severe and non-severe cases, Abraham said.