Doctors should give antiviral drugs to pregnant women and other at-risk groups as soon as they show clinical symptoms of swine flu to prevent them developing serious complications, the World Health Organization said Thursday.

The global body is concerned that some doctors are waiting for lab confirmation of the disease before prescribing antivirals such as Tamiflu to vulnerable groups, a senior WHO medical officer said.

"In order to prevent progression to severe disease, antivirals need to be administered early," Dr. Nikki Shindo told reporters during a conference call.

"This also holds for otherwise healthy people who show progressive symptoms" such as breathing difficulty or prolonged fever, she said. The pandemic strain A(H1N1) can sometimes cause serious illness in young adults — a fact that still puzzles experts as that group is usually the most resistant to flu.

"We have heard that doctors involved in caring for very sick patients in intensive care units regretted that the patients arrived too late and even the most sophisticated medical procedures could not save their lives," said Shindo.

The first 48 hours of illness are considered crucial, but antiviral treatment can be helpful even after that period if the patient is severely ill, she said.

Known risk factors include pregnancy, obesity, asthma and a weakened immune system. WHO estimates that 1 in 25 people in the developing world belong to these at-risk groups, Shindo said. She provided no equivalent figures for developed countries.

Shindo said most people would recover from swine flu without drugs or hospital treatment, and warned against taking antivirals as a preventive measure. Tamiflu and other medicines should never be bought over the Internet, she said

WHO has urged governments to do what they can do minimize the number of people hospitalized with swine flu, as few countries' health systems are equipped to deal with large numbers of severely ill patients.

WHO is already getting reports from Ukraine, Afghanistan and Mongolia that their hospitals are being overwhelmed by pandemic flu, Shindo said.