Dr Zhong: "How can you control it if its origin is unclear"

Zhong Nanshan, director of the Guangzhou Institute of Respiratory Diseases in Guangdong province - where the Severe Acute Respiratory Syndrome (Sars) is believed to have emerged - said it was better to describe the illness as "contained".

China has repeatedly made assurances that the bug is under control.

The World Health Organization (WHO) announced on Friday that it is to beef up its team of investigators in China to try to gain a better understanding of the disease, and develop better strategies to prevent its spread.

So far, the illness has infected at least 2,700 people worldwide, and killed at least 106, according to the WHO.

Common cold link

Studies now strongly suggest that a new strain of Corona virus, which normally causes the common cold, could be to blame.

According to an infectious disease expert, the mystery illness is probably in Asia to stay.

Dr Hughes, the head of infectious diseases at the CDC, said that it was also possible that the illness would become a perennial feature of life in North America.

"In terms of its introduction into North America, whether it is here to stay I think remains to be seen but I think we should assume that it may well be."

His warning comes a week after a colleague, CDC director Dr Julie Gerberding, said that intensive efforts to contain Sars might fail.

The United States Government is recommending that Americans defer non-essential travel to China.

New cases

In Guangdong, 24 new cases were reported between Wednesday and Thursday, bringing the province's total to 1,237.

In Shanghai, a health official and the US Consulate said two Americans and five other foreigners were hospitalised with possible cases of SARS.

The WHO has launched a review of health facilities in Beijing as part of efforts to bring the disease under control.

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Chinese state media reports that nationals from Finland, Canada, Hong Kong, Macau and Taiwan have come down with Sars.

A Finnish official with the International Labour Organisation, Pekka Aro, died of the virus.

On Thursday, friends of a US citizen who died from Sars claimed he had been wheeled, already dead, into an ambulance and sent from Beijing to Hong Kong because China's authorities did not want to report that another foreigner had died of the virus in the city.

The Canadian embassy said one of its nationals is infected but is recovering.

How Severe Acute Respiratory Syndrome has spread around the world SARS guide

Hong Kong reported four more deaths on Thursday, pushing its total past 30 - although the WHO has yet to include these in its latest statistics.

In other developments:

Malaysia continued to block entry of visitors from China and Hong Kong, despite protests from tour operators who feared it would batter the tourism industry.