China has been urged to reveal the full extent of its Sars outbreak amid claims that true case numbers are being concealed.
Corona virus taken from a Sars patient (Image courtesy of the Lancet)
The appeal came after a team of epidemiologists from the World Health Organization ended a six-day tour of Guangdong province in southern China, where the illness is believed to have first appeared.
South Africa has now reported a "probable" Sars case - which, if confirmed by the WHO, would be the first on the continent.
A spokesman asked the Chinese authorities to be transparent about the numbers of people affected by the bug.
"We are always insisting that to address this outbreak you need full and open reporting," said Chris Powell, a spokesman for the organisation.
The Chinese authorities have been accused of a slow and secretive handling of the Sars outbreak in order to avoid spreading panic.
The government says 19 people have been infected in the capital Beijing, with four deaths.
But health workers in the capital have told the BBC that at least 100 people have been infected.
Chinese health authorities announced on Tuesday that the rate of new cases in Guangdong had more than halved in the past month.
However, a Chinese military doctor, Jiang Yanyong, has taken the highly unusual step of publicly contradicting the authorities, claiming that at least nine people had died in Beijing's four military hospitals alone.
The WHO experts also said they expected "huge" progress soon in understanding Sars - Severe Acute Respiratory Syndrome.
The number of people killed by the pneumonia-type virus continues to mount.
According to official figures, 103 people have now died in 32 countries, half of them in China.
The WHO officials spoke optimistically of discovering more about the virus once China supplied specimen samples from patients.
They said a breakthrough could come in weeks.
"I think we've been able to fill in a few more pieces of the puzzle," said Wolfgang Preiser, one of the WHO team.
He also cautioned against over-reaction to the outbreak.
The "probable" South African case follows two false alarms in the country.
It involves a businessman who returned from a trip to Hong Kong then developed Sars-like symptoms.
In a separate development, experts detailed a new theory on how the Sars illness raced through an entire apartment block in Hong Kong earlier this month.
They believe that cockroaches may have carried the infection from flat to flat.
SARS: PROBABLE CASES AND DEATHS
China 1,279 cases (53 deaths)
Hong Kong 928 (25)
Singapore 113 (8)
Vietnam 62 (4)
Canada 91 (10)
Thailand 7 (2)
Malaysia 1 (1)
Source: World Health Organization (14.30GMT Tuesday)
Note: The WHO only records cases and deaths it believes are "probable" Sars - figures from national health authorities may vary.
The cockroach theory was voiced by Hong Kong Deputy Director of Health Leung Pak-yin on Monday.
He was talking about how the disease spread rapidly through an apartment block at Amoy Gardens in Kowloon.
In just a few days, more than 300 new cases arose among residents of the block.
The cases left health officials baffled and deeply concerned, as many of the 300 had had no direct contact with anyone who had Sars.
Leung said: "The drainage may be the reason. It is possible that the cockroaches carried the virus into the homes."
Scientists are increasingly confident that the illness is caused by a virus related to that which causes the common cold.
A team from the University of Hong Kong studied 50 patients with Sars from five separate outbreak clusters.
Nine out of 10 showed evidence of infection by a coronavirus. In contrast, the virus was not found in any healthy people that the scientists also examined.
The scientists, who published their work on The Lancet website on Tuesday, said the virus they had isolated was not one of the two known human coronaviruses. They believe it may have originated in animals.