The deadly form of viral pneumonia that has claimed the lives of at least 58 people around the world may have taken on a new form, according to The World Health Organisation (WHO).
The news comes as the first suspected cases in teh United States were suspected.
Mr David Heymann, head of the WHO's communicable diseases today said outbreaks of the mystery disease are being contained in Vietnam, Singapore, and Canada but Hong Kong and China's Guangdong province remain a major concern.
He also warned the outbreak of Severe Acute Respiratory Syndrome (SARS) in Hong Kong - which accounts for 530 of the 1,633 cases reported since November last year - may have taken on a "new form".
Hong Kong authorities are rushing to see if a cluster of cases in a hotel and quarantined apartment block might have been passed on by water, sewage or ventilation systems.
"We will know tomorrow morning whether or not [the Hong Kong authorities] have been able to confirm that all the cases that they have now are still by person-to-person contact," Mr Heymann told journalists.
But the cluster might equally have been caught from close contact in a lift or from a door handle touched by someone suffering from SARS, he said.
A WHO team in Beijing is still trying to get regional authorities to allow them to go to Guangdong province, the source of the outbreak last November which is thought to account for about 806 cases.
"We believe that an invitation will be coming, and as I say every day we haven't yet received that invitation," Mr Heymann commented.
"We believe that the outbreak is still going on in Guangdong from what the government has told us," he added.
At least 58 people have died of SARS since November 2002, 34 of them in Guangdong, according to WHO.
Meanwhile, US authorities this evening quarantined an American Airlines flight from Tokyo and Hong Kongcarrying around 200 people, after four showed symptoms of the deadly virus.
American Airlines Flight 128 was put under a tight quarantine after landing at San Jose International Airport south of here.
"The aircraft is on the ground and sealed after possiblesymptoms of the disease appeared on board," an airport official said.
American Airlines and Federal Aviation Administration officials could not immediately be reached for comment. AFP