GENEVA - The World Health Organisation said Friday that school closures appear to be the most effective way of preventing the spread of the swine flu virus when implemented early in the outbreak.

While the UN health agency would not issue a definitive guidance on whether or not school closures should be implemented, it said the measure could cut health care demand by up to 50 percent at the peak of the pandemic.

"The timing of school closure is critically important," said the WHO in a note detailing measures that can be taken in schools to reduce the impact of the A(H1N1) pandemic.

"Modelling studies suggest that school closure has its greatest benefits when schools are closed very early in an outbreak, ideally before one percent of the population falls ill," it added.

Its main benefit lies in slowing the spread of the virus. This is most important when the number of people requiring medical care "threatens to saturate or overwhelm health care capacity," said the WHO.

The measure can also buy some time for countries to secure provisions of vaccines and drugs to fight the A(H1N1) virus, from which at least 3,205 people have died since it was uncovered in April. The figure represents a rise of more than a 1,000 on the toll of a fortnight ago.

However, if schools are closed too late into the outbreak, there would be little effect in limiting the spread of the flu, added the WHO.

As schools and health ministries weigh up the benefits of school closure in case of an outbreak, the UN health agency also pointed out that potentially high economic and social costs are tied to such a measure.

Studies show that school closures can lead to the absence of 16 percent of the workforce as parents stay home to take care of their children, it said.

These parents could include doctors and nurses, and this measure could therefore inadvertently bring about a disruption in health care provision at a time when such care is most needed, said the WHO.

Since it was uncovered in April, the new flu virus has swiftly overtaken other flu viruses to become the most prevalent form of influenza.

According to the WHO's flu database, 76 percent of flu detections reported during August 23 to 29 have been found to be of the pandemic strain.

The UN health agency also noted that flu activity appeared to be increasing in some parts of the northern hemisphere, which is about to enter the flu-prone winter season.

Parts of eastern Europe and south-east United States are already reporting increases in infections, said the WHO.

In tropical South America and Asia, the caseload is also increasing. Countries such as Bolivia, Ecuador, Venezuela, India, Bangladesh and Cambodia are all reporting more flu cases, said the WHO.

Meanwhile, in Central America, the Caribbeans and the temperate regions of the southern hemisphere, the number of flu infections appear to be declining, it added.