The World Health Organisation on Tuesday questioned a reported sharp fall in new cases of Severe Acute Respiratory Syndrome in China, the world's worst hit country, saying it was 'concerned' at how the counting was being done.

The United Nations health agency has warned the only way to beat the potentially-fatal respiratory disease, which has no simple treatment, is through rigorous identification of cases and immediate isolation of suspected cases.

It fears that if the disease, which kills 15 percent of its victims, takes root in the vast Chinese countryside, it could become a permanent health threat around the globe where more than 8,300 people have already caught it.

On Tuesday, China announced just three new cases, two of them in Beijing. It has gone nine days in a row with fewer than 10 officially reported fresh infections.

"We are concerned about how these cases are being counted...We do not know enough about where these numbers are coming from," WHO spokesman Iain Simpson said.

"It may simply be that there has been a dramatic drop off in the number of SARS cases, but clearly because of the way that SARS emerged in China, China has a credibility problem," he said.

China, which has over 60 per cent of the world's SARS cases, took four months to inform the WHO about the illness, believed to have begun last November in the south of the country.

Simpson said the WHO was particularly concerned about the situation in capital Beijing, adding its officials were working with Chinese health authorities to gather more information.

More than 5,300 people have been infected with SARS in China and 334 people have died.

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