The World Health Organization (WHO) has expressed concern about the reported sharp drop in new Sars cases in China in recent days.
No let-up in protection against Sars in Beijing
Just three new cases of the respiratory disease were reported on Tuesday, compared with daily tallies well into double figures in recent weeks.
"It raises questions about Sars and how communicable it is and it raises questions about China's reporting of Sars," WHO spokesman Iain Simpson told BBC News Online.
He said it was "quite conceivable" that the outbreak was now coming under control, but the sudden nature of the fall and China's concealment of the spread of the disease in its earlier stages raised questions.
Asian officials attending a Sars-prevention conference in China cited the case of Toronto as an example of the dangers of relaxing prevention measures too early.
It may simply be that there has been a dramatic drop off in the number of Sars cases, but clearly... there is a credibility problem
The Canadian city is experiencing a second outbreak of the disease after it had been declared contained.
On Monday, Toronto officials announced 10 new cases of the disease and one more fatality, bringing its death toll to 32.
"Despite two-and-a-half months of intensive work we know very little about how communicable Sars is.
"The drop in Sars numbers could be something to do with its relationship with the seasons, or some other change in the environment that brings the numbers down," Mr Simpson said.
The WHO had several teams working in China with government medical teams "all trying to ascertain scientific answers", he added.
"It may simply be that there has been a dramatic drop off in the number of Sars cases, but clearly because of the way it was handled when it first emerged in China, there is a credibility problem."
Sars is believed to have started in the southern Chinese province of Guangdong last November but China said nothing until the disease reached Hong Kong and Vietnam in March this year.
China has reported more than 5,300 cases, with 332 deaths.
A top Chinese health official has said the government would "not let down its guard" in the fight against the disease.
"We must maintain sharp vigilance and continue the prevention and treatment initiatives unremittingly," said Gao Qiang, China's executive deputy health minister.
He was speaking at a Sars-prevention conference in China attended by South-East Asian, Japanese and South Korean officials.
"We are fully aware that the fight against Sars in China is far from over, " he said.