David Heymann, the WHO's chief of communicable diseases, said, "China is the key and it's the unknown question in the whole formula, because if China cannot contain it, then it can't be removed."
The WHO's concerns about the situation in China have reportedly been exacerbated by the lack of information coming out of the country's health care system.
The Chinese government's decision to begin daily reports of new cases and deaths from the disease less than two weeks ago was only a start, WHO China representative Henk Bekedam said Monday.
To judge how Beijing's health system was holding up under the epidemic and what preventive measures were effective, the WHO needed details, like the types of cases, where they were occurring and who was getting sick, Bekedam told a news conference.
"We do think that it is now high time that this information becomes available," he said. "This information is also crucial for everybody to have an understanding from what's going on and I think at this very moment we don't know."
Nine days ago, before China began open reporting, Beijing officially had just 37 cases and four deaths from SARS. On Monday, 96 new cases were reported in Beijing, taking the total cases to 1,199 and total deaths to 59.
"The key question is what do we know about the 96? Who are the 96? When did they start? Where did they live? This kind of data at this moment is the big challenge," Bekedam said.
Despite its late commitment to disclosing such figures, China has mounted a massive campaign to stem the spread of SARS and ordered the closure of schools and public entertainment venues, including theatres, Internet cafes and karaoke parlors in Beijing.
Newspapers said 7,672 of the capital's 14 million people were under strict quarantine after the city enacted sweeping powers to control the flu-like virus.
On Sunday, WHO officials urged Beijing's top officials to tell the public more about how to fight SARS to stem the panic. Beijing responded with a pledge that it would say publicly how many SARS cases were at each hospital and would report which buildings and how many people had been quarantined.
In sharp contrast to China's public reaction to the SARS outbreak, Vietnam worked quickly to contain the virus. Sixty-three people have contracted the virus in Vietnam. However, a move to cordon off the Hanoi French Hospital on March 11 has been credited with slowing the rate of infection and keeping SARS from spreading beyond its doors.
No new SARS cases have been reported in Vietnam since April 8. The WHO has set a 20-day window -- double the disease's reported incubation period -- as the standard for lifting travel advisories and declaring that an outbreak is no longer spreading in a region.
Other Asian governments have also been using quarantines and travel restrictions in their fight against SARS. Taiwan on Monday began enforcing a 10-day quarantine for visitors arriving from areas hit hard by the virus, prompting airlines to cancel some flights there, while Malaysia sealed off a hospital in Kuching it fears may be the site of an outbreak.
Meanwhile, Canada is contesting a travel advisory from the World Health Organization that also warned people to postpone trips to Hong Kong and parts of China.
WHO spokesman Dick Thompson said the organization would review its advice to travelers on Tuesday, but offered no assurances.
"We have received a lot of information from Health Canada and we are reviewing it," Thomson told Reuters. "It is possible we may change it [the warning]. It is possible we may not."