The WHO team stopped short of backing cover-up allegations by others, but said the Chinese capital could have as many as 200 "probable cases" of severe acute respiratory syndrome — far more than the 37, including four deaths, that had been revealed publicly.
They said the Chinese military had failed to report SARS cases in its hospitals.
The global death count rose to at least 161 Wednesday with five new fatalities in Hong Kong and one each Singapore and in the southern Chinese province of Guangdong, the likely birthplace of SARS.
Scientists Wednesday said they had confirmed the identity of the virus that causes the lethal new disease known as severe acute respiratory syndrome, the World Health Organization said Wednesday.
In experiments conducted at Erasmus University in Rotterdam, Netherlands, researchers infected monkeys with the coronavirus suspected of causing of SARS and found that the animals developed the same symptoms of the disease that humans do.
Scientists were almost certain that a new form of coronavirus first isolated from sick patients on March 27 by the University of Hong Kong was the cause of SARS.
But they could not say for sure until they had satisfied what is known as the Koch's postulates — four scientific tests that verify whether a bug causes a certain disease. That has now been done.
In Hong Kong, three babies have been delivered by Caesarean section after their mothers were badly infected.
On Wednesday the infants had breathing problems and two had fevers — symptoms that point to SARS, although they have tested negative for the virus.
To contain the spread of SARS, passengers departing Hong Kong will soon have their temperatures taken at the airport. Those with SARS symptoms will be kept off flights, reports CBS News Correspondent Barry Petersen
That's more bad news for airlines flying their big jets with as few as three passengers. The main airline here, Cathy Pacific, may ground its entire fleet if it gets much worse.
Malaysia and Saudi Arabia are banning everyone from SARS infected regions. The Philippines is asking them voluntarily to just stay away.
Meanwhile, tourists and business people staying away from Hong Kong are creating a litany of economic woe.
Hotels more than 90 percent empty, and 5,000 restaurants and 10,000 small stores are facing bankruptcy.
Even civil liberties may suffer, reports Petersen. One human rights advocate is now suggesting that families of SARS patients be rounded up, and sent to quarantine camps in the hills surrounding Hong Kong.
The virus nonetheless is "something that is new to science," university microbiologist Malik Peiris said before the WHO findings were announced.
Asked about the possibility that the virus was man-made, Peiris said there was no chance of that.
"That whole genome is essentially new," he said. "Nature has been the terrorist throwing up this virus."
"What's dangerous about this is we don't know its potential," said Dr. David Heymann, head of WHO's Communicable Diseases Cluster. "For the present, everything hinges on what we find out in China, as far as our projections."