H1N1 influenza is causing mild symptoms that go away without medication in most patients, but care-givers should be alert for warning signs of severe cases, the head of the World Health Organisation said on Thursday.

"The overwhelming majority of patients experience mild symptoms and make a full recovery within a week, often in the absence of any form of medical treatment," WHO Director-General Margaret Chan told a conference in Cancun, Mexico.

"But there are some exceptions that must be the focus of particular concern," she told the meeting, according to the text of her speech distributed by the WHO in Geneva.

Pregnant women and people with underlying health problems are at higher risk from complications from the virus and need to be monitored if they fall ill, Chan said.

Those with normal flu-like symptoms should not seek care in hospitals unless they have certain "warning signs" of severe infection, the former Hong Kong health director stressed.

Adults with a high fever that lasts for more than three days should seek help, and children who have difficulty waking up, are lethargic or are no longer alert may also need extra care, Chan said.

"For a pandemic of moderate severity, this is one of our greatest challenges: helping people to understand when they do not need to worry, and when they do need to seek urgent care," she told the conference, also attended by Mexican Health Minister Jose Angel Cordova.

Last month the U.N. agency raised its pandemic flu alert to phase 6 on a six-point scale, declaring that the H1N1 virus, commonly known as swine flu, was causing the first influenza pandemic since 1968.

Worldwide, more than 79,000 cases of H1N1 flu have been confirmed, and 337 people have died -- most of them with other underlying health problems -- since the disease was first detected in April.